I notice that Randy was only in a few episodes in the entire series, but I don't remember him being a huge part of the show. Was Kevin friends with him or was he just a tag a long guy from school?

Randy Mitchell?

A background character in the early episodes with a few (mainly smart ass) lines. A fellow class member that sometimes gave Kevin a hard time and was your typical mouthy kid. He was quite bossy towards Kevin in the "Miss Haycock - Glee Club episode".

The writers mellowed his return in the 4th series even to the point where Kevin gave HIM attitude back during the "Night Out" episode. When they were all inside Walter's garage at night Randy's silly suggestion for the guys to make Owl noises as a "signal" resulted in Walter's furious Mother catching her Son sneaking back into the House to steal Beer and sent him straight to bed.

By the later High School seasons he appeared on and off as a member of Kevin's inner circle of friend's but not quite the smart ass anymore.

Yeah I remember Randy being more outspoken and outgoing in the early seasons like in season 2 he made fun of Kevin teased on him.