So, this little centipede just kinda randomly fell on Vandalieu's head while he was roaming the forest and just kinda burrowed in him and stayed in there. Van gives him a name, kinda just takes him in, and sticks around by staying in his hair/head (not sure if it actually burrowed into his head, but it's definitely in his hair).
Since Pete is still a monster, it has fighting capabilities. Since Pete, when it's still small, is inside Van's hair, it can get a sneak attack in unexpectedly because typically you forget that he has that bug in his hair. And just like every subordinate and friends of Van, he levels up and eventually goes up in rank. Just like the starting skeletons who began as rank 1 and are where they're at now, ferocious warriors.
And because he sticks around where the action is, right in Van's hair, Pete gets front row seat in the action and in the exp farming. Eventually that guy becomes gigantic and can no longer just stick around in Van's hair, but by that time it's not an issue since there are other method of transportation anyways. If I recall right, Pete also ends up being very lightning focused. The part where I kinda stopped reading, they were actively waging wars against the gods and were fighting demi-gods and dragons. I forget if Pete was rank 11, 12 or 13. But from memory, Demigods are around Rank 12. Mind you, I have not read this in months, so my memory's a bit spotty here and there.
Do keep in mind. Pete is still an insect, it does not possess the ability to speak. So Pete really just appears mainly for combat and doesn't really show up much if at all durinng the slow life moments