A heavy set kid his whole life, Jason was always bullied and pushed around by other kids when he was younger. It only stopped when he remembered the old adage, "If you can't beat 'em, join 'em" and that's exactly what he did when he started to pick on the kids that were smaller than him. It worked surprisingly well and he soon found himself running with a rough crowd which caused him to sick further and further into delinquent behavior.
By the time he entered high school he had already spent time in juvenile hall and had quite a rap sheet. He had also vandalized the school quite a few times and in one instance managed to shove one of the smaller kids into his locker.
Of course he did have a bit of help in these acts from not only his buddies from previous years but also Morgan and Jon who he met on his second time through freshmen year. Though while Morgan was more than happy to help him intimidate other kids, Jon was a little more hesitant and preferred just to cut class, back talk teachers and maybe do some damage to the school.
This divide was only widened when they all entered the Digital World and Jason and Morgan ended up on Serpentmon's group while John joined The Guardian's Digidestined. The duo also showed their true colors when they had no problem turning on their so called friend when push came to shove.
Though Jason was a little sad about loosing his friend, he got over it as he got to know his new Digimon partner Kunemon more and more. At first he was upset at having the weak looking bug-like Digimon as his parnter but after he received a few powerful shocks from said bug he grew to not only like him but respect him a bit. While Kunemon himself is still getting used to being around the human he does appreciate Jason's mean streak and cruelty and believes that they may just get along yet. Even if the bonding is just through the suffering of other Digimon.