+ Def -Spd Legault - Is there anything I can do to help him?

I personally prefer Smoke Dagger. I never really found his The Cleaner to be all that.... reliable, to be honest. It's too heavy on the "the enemy needs buffs" thing, and there really is no way to actually buff your enemies, or at least no way that I can think of.

Legault is a dagger unit, so his main role, like most daggers, is more "support" and utility. Smoke Dagger can debuff all of the enemy's stats by 6 (and it's an AoE effect), allowing your other squad member's to have an easier time dealing with them.

Or..

If you do keep his The Cleaner, then you can debuff the enemy's defense and resistance by 7, then give him Atk Smoke C and Spd Smoke Seal, allowing him to pull off a faux Smoke Dagger while keeping his buff-slaying effect.

He is definitely playable. He's so fast to begin with that the bane leaves him at above-average speed. To demonstrate the point, he starts with the same speed as neutral B.Cordelia/S.Takumi, and he's faster than neutral Sothe, Saizo, and Kaze.

You need to consider what teams he might want to be on. He can still play a windsweep build, but with the spd bane there are better candidates. He can become quite good at baiting enemy daggers and archers (most notably B.Lyn, who has poor defenses) by pushing his speed just high enough to block their double and then doubling them back with QR and throwing a moonbow in their face (or bonfire with barb shuriken).

If you like his native weapon, you can set him up specifically to put the hurt on emblem bows and daggers. Use atk/def bond and atk+3 seal. When they hit you with a hone or fortify, you counter with 63 atk, which is fairly likely to bust them in one shot. I prefer barb shuriken + glimmer, but if you don't have the feathers to fodder that way, this is probably the best one-shot attempt.

Alternatively, you can just say "I have moderately good def" and start firing away at people, use brazen atk/spd and desperation, survive the first hit because you have 27 def, and enter desperation.

One interesting hybrid build made possible by the introduction of starfish is desperation + QR without hogging the QR seal. I can't say I especially like this because Legault doesn't hit that hard, but it's a decent build. Starfish+spd, moonbow, brazen atk/spd, QR, flashing blade once the tempest trials begin. The point is that you bait some dagger/bow, block their double, double them back, and now your brazen is active and you go on a rampage with 50 atk 44 spd, and everybody who you double gets a moonbow.

One last option is a build I saw on a Kagero who completed a solo of the Linus GHB: rogue dagger+def, moonbow/bonfire, close counter, vantage (could substitute desperation/QR/anything else), atk smoke, close def. Debuff them, achieve 42 def at melee range (49 if you count atk smoke), close counter everybody to death.