During the original series, Ben and Gwen were almost always bickering, arguing, or fighting, and they had a love/hate relationship. However, deep down they really do care about each other as shown in Secret of The Omnitrix, when Ben would go far lengths to keep Gwen out of danger. such as turning into Four Arms, despite the fact that the Omnitrix's self-destruct mode countdown would only accelerate. When he found out that she was still alive, he ran through the open field of heavy fire in order to hug her.
In Alien Force, they have stopped their hostile fights are each other's favorite cousins respectively. In Alone Together, after Ben is teleported safely back to Earth from a desert planet, Gwen runs up to and embraces him which he happily returns. In Time Heals, Ben once more reveals his caring attitude towards Gwen after she frees him from a torture chamber created by Hex or Charmcaster. He states that he "already lost her once and doesn't want to lose her again." He then buys Gwen some time in order to allow Gwen to reverse her faults and restore time back to the way it was.
In Ultimate Alien, their relationship has once more received an upgrade, as shown in Fused, when Gwen painfully watches in horror as Aggregor seemingly kills Ben as she is worried about his safety. In Girl Trouble, when Gwen is attacked by Sunny and Antonio, Ben immediately steps in and helps fend them off stating that no one can pick on Gwen except him. In Greetings from Techadon, while at Mr. Smoothy, both of them talk about how they were when they were ten, and how they used to not get along, and then soon laugh at the things they did as younger kids. In The Ultimate Sacrifice, they have bonded so much that he kisses Gwen goodbye on her forehead, something the 10 year old Ben wouldn't do, before jumping into the fire pit to free his original Ultimate Forms.
In Omniverse though Gwen went to college early, both have openly stated that they miss each other (hugging one another before she left in The More Things Change: Part 1) and on occasion said they love each other. Gwen runs to hug Ben when he returns to earth in The Frogs of War: Part 2. Ben also has stated to Gwen that she should try to make more time for them to hang out more whenever she visits him from college in Mud Is Thicker Than Water, but in Mystery, Incorporeal Gwen is annoyed that Ben got an honorary degree when her college was having an open house while she was still studying there herself. This means that while they still love each other very much, they like to get on each other's nerves for old time's sake, sometimes without trying.
In the original series, Ben's relationship with Max seems to be that of a father and son, because the majority of the series took place in the summer and Max was the only guardian available. When he was seriously injured in The Alliance, Ben went full strength with Four Arms and brutally destroyed Vilgax's robot drones as they were the catalyst for his injury. His relationship with his grandfather has not changed much at all during Alien Force, Ultimate Alien, and Omniverse.
Carl and Sandra Tennyson
Ben has stated that he loves and cares about his mom and dad, and that they're good parents and raised him well enough to become the person he is today.
At first, Ben didn't like the fact of having a new partner, and felt like Rook was just trying to replace Gwen and Kevin. However, he soon began to like Rook. In A Jolt from the Past, Ben seems to care a little about him as he shouted his name when Fistrick knocked him out. In Of Predators and Prey: Part 1, they have their first fight when arguing over which plan to carry out to find Zed. Ben even went as far to say he never wanted Rook, but this was revealed to be a ruse to make Khyber show himself. They eventually work well together with their respective strengths covering each other's weaknesses, and they share a good camaraderie and love for smoothies. In Many Happy Returns, Ben states, "I don't always get him, but he's cool", to which Rook replied that the feeling was mutual. Ben already considers Rook to be his best friend, only after Kevin and Gwen.
Ben's initial thoughts of Azmuth before meeting him was never stated, but he did find it to be somewhat ironic that he was a Galvan, despite the fact that he's seen what Galvans can do when he's Grey Matter. After meeting him, he thought of him as a jerk who cared about no one but himself. However, his attitude changes when Azmuth fixes the Omnitrix.
In Alien Force, Ben meets Azmuth again only when the situation was important (e.g. War of the Worlds). Ben sees him as another mentor and follows his advice, at least until Season 3, when Ben loses Azmuth's trust in the second episode.
Ben sees Paradox as a guide when Azmuth or Grandpa Max isn't around. Throughout the series, Ben has received advice from Paradox about upcoming future events, even ones that that are possible.
Ben met Rex in Ben 10-Generator Rex: Heroes United, mistaking him for an alien, and Rex mistaking Ben as an EVO. The two continued to be enemies until they both realized who the real enemy was. They eventually share a friendly bond and explain how they came to be who they are. Once Ben leaves, he is hoping to see Rex again one day and vice versa.
Particularly because of Tetrax being Diamondhead's species, they have shown help to each other, such as in the episode Hunted.
Elena Validus is one of the Plumbers' kids and a former member of Ben's soccer team. She is mentioned by Gwen as Ben's first crush, though this contradicts what was seen in the original series, as Ben met Elena when he was thirteen, while he met Kai when he was ten, but Kai never actually liked Ben. Rook also says his first crush was Patty Berkenfeld. Ben had to convince his soccer teammates to let her join. She appeared in the live-action film Ben 10: Alien Swarm.
Elena also appeared in Revenge of the Swarm, where she served as the Queen of the Hive. Victor Validus was attacking Ben at night, so the trio went to her to find whether she knew anything about the Nanochips. Later, when Ben mentioned Julie, it made her angry and jealous, and kidnapped her in order to keep Ben all to herself. They almost kissed, but her plan was revealed to Ben and he saved Julie. Elena, being possessed by the Hive Queen, encountered Armodrillo and choked him with the nanochips. However, she was convinced by Julie to not to do so if she had really liked Ben, and the true Elena, that still cared for Ben, willingly sacrificed herself in order save Ben, proving how much she liked him and that there was still the Elena that cared for him. Ben became sorry for her, understanding how much she had liked him. At the end of the episode, the nanochips reform, revealing she is still alive.
Elena appeared in The Perfect Girlfriend disguised as Julie. As Julie, she tried to do everything to make Ben happy. Once Ben finds out Julie is Elena, he tried to reason with her and fought her as Ultimate Echo Echo, but lost after she choked him with Nanochips again. Julie tells Elena that if she kills him, then no one can have him. At the end of the episode, Elena retreats into the sewer, "knowing what hate feels like" and promises to return.
Jennifer Nocturne is a famous film actress who appeared in Hero Time, when she had an instant crush on Ben when he saved her, then kisses him from the lips in public. Later, she tried to make up a relationship with Ben for publicity and to make a better career for herself. Later on, she was captured along with Julie by Captain Nemesis, who later called himself Overlord. When Overlord had the two of them hanging from a statue, he gave Ben the option to save one of them before blasting the cord holding them. Without any hesitation, Goop saved Julie while Kevin and Gwen saved Jennifer.
Ben and Kevin started flirting with Eunice and she stated that Ben was bad at flirting. In the end, Ben let her go because she is the Unitrix and quite possibly dangerous, and also due to having Gwen's randomized DNA which makes her technically related to him. Azmuth, at Ben's insistence, decided to compromise and have her be Azmuth's assistant. Before Eunice leaves with Azmuth, she gives Ben a blue flower as a reminder of her. In Simian Says, Eunice kissed Ben on the cheek.
Emily briefly dated Ben, but the reason they broke up was that Ben went "hero" and left Emily on a 200 foot tall watchtower, causing Emily to hate Ben.
When Ester first met Ben in Hot Stretch, they fought for the fusion device. Later when she realized her kind was doing evil things, she joins Ben's team. She brought him to her friends' town and they played an alien version of hockey. Ben turned into Water Hazard and splashed everyone with water to cool them down, even when Ester told him not to. He then brought her a towel, and she blushed as he gazed at her. She turned the water floating around him into snowflakes and ran off laughing. Rook told Ben she liked him. Later when she became the chief, she hugged Ben and told him she and her kind would be good neighbors. Later Ester's friends called Ben her boyfriend, and Ben at first denied it, but then excitedly asked Rook if he had a chance with her.
In Rules of Engagement, Ben offered to help clean up the Hot Spot with her. She excitedly agreed, wrapping her arms around his arm. She also stated that she is fond of Mr. Smoothy.
Sometime before Return to Forever, they went on a movie date.
In Fight at the Museum, Ester invited Ben to the museum for a date, but Ben, not knowing it was a date, brought Rook since Rook enjoys museums. Ester is shown to be horrified when she hears that Ben and Kai end up together in the future Spanner comes from, while Ben himself states there's no way that's going to happen.
Later in The Most Dangerous Game Show, feeling that Ben really was destined to be with Kai, Ester decided to let go of him. While trying to talk to Ben, who she couldn't get in contact with, Ester ultimately fell in love with Antonio. Eventually, Ester made her feelings known to Ben, breaking up on good terms, even stating she still likes him while remaining friends.
In Many Happy Returns, when Kevin was about to marry her, Ben challenges Looma to a battle to stop it. She keeps defeating him until he turns into Four Arms. When Four Arms defeats her, it is revealed that female Tetramands marry the ones that defeat them in battle. Kevin and Gwen leave, and Looma says she'll be back in three years. Ben mutters that Kevin is "so dead".
In Rules of Engagement, Looma returns, attempting to collect four items for their wedding: something conquered, something bruised, something severed, and something blue. Looma battled the Kraaho, the tribe of Ester, to capture one of them as something conquered, and Ester asked Ben for help. Looma was happy to see Ben again, but was busy with collecting the items. Ben tried to stop her, but she tossed him aside and took Ester, Rook, Julie, and even Hervé, for being a photographer. She also captured Fistina's right hand, which leads Fistina and Ben to get it and "little Rook" back. Looma went to Ben's house and was happy to see her future "mother-in-law" and presented her "offering gifts". When the "offering gifts" tried to escape, Looma tried to re-capture them. Looma fought Julie when she heard that she is Ben's girlfriend, not knowing that they broke up. With the help of her friends, Julie was able to defeat Looma. Looma was saddened, but accepted her defeat. However, Looma promised Julie to crush her skull the next time her and Julie meet again.
In Showdown: Part 1, Ben says that his first crush was Patty Berkenfeld from 3rd grade.
In The Frogs of War: Part 2, Bullfrag infiltrates her team and from the moment she saw him, Attea had a crush on him. When they meet in the cell block of Plumbers HQ, Bullfrag comes up to her and starts flirting, taking note that his new form shows her in a whole new light. Attea is a bit flustered, but starts flirting back, only to be knocked out by a mana blast by Gwen. Bullfrag says that happened because he was being influenced by the Incursean DNA.
At the end of the episode when leaving Earth, she gives Ben the offer to come with her and "Ditch that monkey face and turn into Bullfrag again" He promptly declines. Attea gives him a frog-like kiss to the back of his head with her long tongue, winks, and takes off.
She later decides that Ben is her only option for a mate in Catfight, however Ben later convinces her that she is capable of ruling on her own.
In T.G.I.S., Ben gains a crush on Drew Saturday the minute he saw her and nervously tells her, "Meeting you is the good", to which Drew responds "Right back at you". When Doc tells Ben, Rook, Zak, and Fisk that Dr. Animo and his Chupacabra army are attacking Mr. Baumann's store, Drew offers to stay behind to look after the petrified alien. Ben though suggests that Rook stay behind and Drew come with him, telling them that they might need her medical expertise. Later, when Ben, Rook, Zak, and Fisk arrive at the airship to stop Argost and his army of cryptids, Doc and Drew reveal themselves and tell them that they escaped moments before Argost showed up. Ben then instantly tells Drew that he's happy that she's okay, causing Zak to tell him that Doc is okay too. At the end of the episode, Ben tells Zak how it was a pleasure to work with him and his mom. He then takes a photo with Zak, Doc, Drew, and Fisk as Four Arms.
Drew knows about his crush and finds it cute and amusing, but Zak doesn't appreciate his crush, but still remains his friend.
Ben has a rivalry with Vilgax ever since the original series, but has shown to be helpful for him when Vilgax is in desperate need of help.
Albedo has a great hatred for the team, especially Ben, but he doesn't hate Albedo as much. In Trouble Helix, it is revealed that they were once allies. Albedo does not hate Ben nearly as much as he hates Azmuth.
Ben used to underestimate Animo until the original series' 4th season. Once he appeared in Alien Force, Ben saw him as a threat, seeing the condition of the Null Void. In Ultimate Alien, he stated that years ago, he thought his plans were dumb. In It's Not Easy Being Gwen, he underestimated him again, forcing him to call Gwen for help.
Mr. Baumann is Ben's old neighbor and they are shown to have a mutual dislike for each other. Mr. Baumann hates Ben, constantly accusing him of destroying his car and other property. However, deep down, Mr. Baumann does not truly hate Ben.
Pakmar hates Ben because he always breaks Pakmar's things. Ben doesn't hate as much as Pakmar hates him, but Ben still doesn't like him because, like Mr. Baumann, Pakmar tends to blame Ben for destruction that wasn't his fault.