Poison Dagger + on speedy assassins?

No. Poison dagger is only good on units with high attack. Even with the eff. multiplier, it can be hard to break the defenses of dragons(I'm assuming you're gunning for a dragon killer here, seeing as they've become meta for defensive teams).

I suppose it could be useful as a mage killer, but we have Kitty Paddle for that.

Gaius' attack isn't necessarily the highest, and now that Sothe exists, Kagero is arguably no longer the best at her own default set, lol. You'd want a unit with high attack to compensate for Poison Dagger's extremely low might (Poison Dagger has 2 might while Poison Dagger+ has 5... my lord) so someone like Kagero, who has 38 attack with boon is a good candidate.

Olivia isn't necessarily a good candidate either considering she has less might than Gaius if I recall correctly.

Again... Sothe is arguably better, but not everyone has him.

I wouldn’t give it Olivia because her attack is so low but with life and death and a +atk nature, gauis can maybe do something with a poison dagger. It’s a build I’ve looked into before but dropped because I already have a lot of dagger units, but gaius is pretty dang fast and life and death helps his attack so it’s a possibility but I’m not sure how high his attack can hit exactly

I saw someone on YT with a +Atk +5 PAOlivia with Death Blow and Poison Dagger. I think with summoner support and Atk+3 seal added to those numbers she sneaks up to 49 Atk upon initiation, but taking away the +Atk nature and 5 merges, you're back on earth again with 44 upon initiation. And this was when she came out, not with all the dragons in arena now.

I never got a Kagero in my 9 months of playing so IDK lol