Good Defensive Blue Units

Hardin, Kliff, Lukas, Shiro, B!lucina, Robin, a lot of people fam.

Catria. If only Null Folllow-Up...
Hopefully they put some thought into a prf for her (she needs something more defensive to set her apart from Sumia).

I'd suggest Lukas. Sure, he's slow, but he hits hard and takes little physical damage. You can easily stack his Defense to 50 and give him Ignis. He's also obtainable at 3*-4*, so he's quite common.
This is my build for him.

Oh good, a post that lets me show off my pride and joy, or at least one of them.

Nowi for sure. Especially if you are looking for all around protection to include Res.

This thread is missing Silas. He's horse Lukas!

Silas is one of my favorite units with a flexible B that I use breakers, and seals on. On Cav team he excels. There are so many though. Oboro, Nowi, Lukas, Shiro.