Am i missing something?

Olwen
Atk: 41

Leo
Res: 37

without triangle advantage you are dealing 4 damage. i think thats swift sparrow or something in her a slot but whatever, im not hardcore mathing it but the point is that you've overlooked Leo's massive resistance vs your fairly weak attack.

I put this is a calculator and Olwen still does 12x4, I'm not surprised since the Leo is buffed up to 37 res

The game is not glitched. Leo has 37 Res, while Olwen has 41 Atk. Triangle advantage buffs this by 20%, or 8 points, putting her at 49 Atk. 49-37=12

The damage from triangle advantage is 8 so that would be it I guess unless triangle advantage is calculated after you minus the attack

Olwen is extremely weak compared to her brother. I have Glacies on her, which is probably an uncommon choice for her, let alone any hero to have. When Olwen procs Glacies, she can, at best, maim her opponents, but can kill a sword opponent with no issues.

Olwen is by all means a niche character, able sponge a lot of magic due to her resistance and A passive, and can also use Reposition and help any one of her team escape. I find her generally weak unless she is among cavalry, and at best, she is adequate enough by herself, only able to KO low resistance reds if she's lucky.

I have neutral, which... is actually kind of bad for someone who needs a boon for Spd, Res, or Attk really.

She can in rare cases quad enemies, and having a speed nature along with a offensive special makes her hit hard one out of four of her attacks, which is unreliable needless to say.

Why she's not hitting hard is probably just due to an opponent having high resistance and by the looks of the map? They may be on guard tiles to make matter even worse.