Shiro v. Lukas

In my opinion, the only things that make Shiro slightly superior are his legendary weapon and maybe his BST (if we compare their unmerged versions). He’s good and you have him with a very good IV spread so if you’re not willing to invest time and feathers, keep Shiro. He’s perfectly fine.
However, what you need to consider about Lukas is his merge potential. If you want a +10 physical tank out of those two, Lukas is the better choice, for obvious reasons.
So it really depends on what you need here. If you want a supermerged blue tank, pick Lukas. Otherwise, just keep Shiro. They are very very similar.

I'd take Lukas easy. Some would argue shiro has a better weapon and stats, but having a legendary weapon can be restricting sometimes; it can be hard to justify changing them.

Mainly though, Lukas is available. You can invest in a Lukas much more easily than shiro.

If I had the option of both with optimal ivs and +10 5, I'd take shiro.

Well Lukas is more specialized as a physical tank with higher hp and def while Shiro has more points in res and spd which are potentially useless, so Lukas is better as a Min/Max unit with a defined. He is also easier to get to +10 because he is available as 4*, but has lower bst

TL;DR Lukas is the better physical tank and easier to +10, making him overall the better unit for his role

With the inheritance on Shiro, I'd say to stick with him. Lukas will want to be + def if he wants to tank, but his lack of spd and res and ok-ish atk leave him in my opinion, worse than Shiro, even though they most likely will be murdered by a small green haired child with access to the knife drawer. You may want to use something like quick riposte on Shiro to help with charging his special. you could also try Aether as his special to help him sustain his hp.

Hoped this helped!
-R

I use Lukas over Shiro. He has a Killer weapon, tanky defense (duh), and really decent attack. And when you slide an Ignis on him? He deals a TON of damage.

Fortress Def is honestly still really good on Lukas. I’ve kept it and I love having it. :)

my lukas with atk def bond, quick riposte, and close def is god

Lukas is my go to guy.