He's really stoic. He considers himself to be more of a scientist than a super hero and is only really urged to be a super hero when there's a crisis scenario where he can be of help. He is uncomfortable with violence and danger. He's bi-polar and often forgets to medicate himself when he's under stress. He suffers from anxiety and feelings of inadequacy, and has a bad habit of addling himself with feelings of guilt over every bad thing that's ever happened that he was even remotely involved in.
In the comics, most of that stuff came to a head in two separate psychotic episodes in as many years (in-universe), the later involving the infamous wife-punching (as well as attempting to kill a super villain who was surrendering and letting a killer robot loose on Avengers Mansion in a deluded Munchausen-By-Proxy scheme to prove that he's a hero, but people tend to forget that part). After that his marriage fell apart, he left the team, became a homeless drunk for a while, then sought psychiatric treatment, cleaned himself up, returned to the team to serve as mission control for the West Coast Avengers, and slowly reconciled his relationship with Janet as he put his life back together.
But that wouldn't happen yet in the movies obviously, so in the film canon it'd probably boil down to "shy idealistic intellectual struggling with mental illness."
t:
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