I know who I am, what I'm good at, what I can and can't do. I'm just... I'm what I am. A great shooter, a bad gambler. Why can't that be enough?
At first glance, Jesper seems your stereotypical cocky, witty, but ultimately good-natured kind of guy. He is an exceptional sharp-shooter, and multiple characters comment on his considerable talent with a gun. Unfortunately, he is also a gambling addict, something which he comes to terms with as the books go on.
However, as he discovers more about his own abilities (no spoilers, but it’s a pretty big deal), and the stakes are raised in regard to the group’s mission, a more serious and sensitive side is revealed to the reader. When anxious he can become restless and brash, and when his friends are threatened, he will do anything to help them no matter the cost.
A shameless flirt, Jesper is shown to find both men and women attractive, and his bisexuality is not seen as "different" by any of the characters, implying that queerness is the norm in the "Grishaverse". Throughout the books he develops a romantic relationship with Wylan van Eck, his total opposite in terms of personality — but you know what they say, opposites attract!
His alternate-timeline self in Netflix’s Shadow and Bone (portrayed by the handsome Kit Young) is much the same in terms of personality, and many fans of the book series praised Young for his performance being so faithful to the character they know and love.