Kevin was originally a villain when he was 11, but when he turned 16, he became a hero and joined Ben. He developed a relationship with Gwen. He has helped on many missions. He defeated many villains, he even defeated personal villains such as Ragnarok and Aggregor. Despite becoming a hero, Kevin has went back to being a villain many times.
Ben 10 & The Master Assassins
In Enter Assassins, Kevin returns to Bellwood with Gwen. They wanted to help Ben against The Master Assassins, and Gwen wanted to return to high school as well. Kevin goes with everyone to search for the assassins, but much to everyone's dismay, they had been tricked and the assassins killed Magister Patelliday.
Kevin does not have that much of a prescense throughout the rest of the series, although, he is targeted in The Osmosian Target. He defeats the assassin.
In the finale, he convinces Gwen to leave Ben alone after he supposedly turns evil.
Crisis: Broken Days
Kevin is confirmed to play a very large role in this sequel.