What Hero you initially thought was completely bad but later found out was decent or strong.

Mine would be butterfly, at first I thought she would be Paquito that gives stack per skill cast then strengthens and resets the enhanced skill, but then at first try I was disappointed that the stacks came from kills. Since she was one of the first heroes tried in playing the game I played her once where I realize where the stacks came from and lost pretty badly and didn't bother reading her skills.

I thought to myself that she might be the worst heroes since initially her skills lack mobility much and effects, her kit also seems like it's pure damage.

Later I found out that each of her skills actually has a ton of effects added that didn't seem obvious, like the Crit up, DMG reduct, ult resets per kill, heal, cd reduce per attack etc.

And when I finally knew how to play her, she's actually incredibly fast at jungle and strong when played right.