meh leif

Leif with a Defense boon has really good physical bulk for a speedy unit. He has a base 36 (with Light Brand), that can be bolstered to 39 with Fury 3. B skill can be Desperation/Wrath for offensive, and Quick Reposte/Guard for defensive. Finally, you can give him an Attack +3 sacred seal if running QR, or Speed +3 for Desperation.
I don't think Leif has the best fodder, but do you happen to have a unit who would appreciate Steady Blow?

With his speed still in tact, Special Spiral is a good option given how naturally speedy Leif is. Though if you don't plan to make much use out of him, a "less premium skill" like vantage, guard, or wings of mercy would fit his kit well enough.

This is how I've built my Leif but with -atk (instead of +atk like below) you could try Slaying Edge/Wo Dao/Light Brand with Bonfire for special as his defense is solid off to add some nice damage and he still has at worst 38 speed which is good. Desperation is good if you wanted to go Fury and a speed refined sword so that he hits 44 spd but you could also build him tankier with the +def and given him like renewal for sustain or swordbreaker to combat/tank other fast swords. Unfortunately I've really only used my below build but hopefully some of the ideas help you in some way. As I feel Leif's fodder potentially isn't that great compared to his overall solid Atk/Spd/Def stats that can help him as both an offensive sword or a more physically tanky sword.

With Nina's bow having the same effect as Light Brand, he might be getting a refine soon.
So it'd be quite a bad idea to fodder him, and at least his boon is pretty nice