Ben Tennyson and Gwen Tennyson

Ben Tennyson and Gwen Tennyson have one of the closest relationships in the franchise.

Ben 10
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.

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.

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.

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.