Female Morgan

Maybe love abounds lyn. With blarserpent she has 33 def and 40 res which is great though I don't have her so I can't say for sure. Also a few people have given Oliver blarserpent so maybe you could try it on him.

There aren't a lot of units that would benefit from serpemt tomes right now. As someome already said LA!Lyn for more bulk. Oliver is also an option however unless you really like Oliver, I couldn't suggest it. Her father might appreciate it since he is very much a defensive unit, and it would help him with his ability to tank magic hits as well, but there are likely better option like an Owl tome or his base tome. Depnds on how you intend to build your units.
Other than that she has some pretty nifty skills: Guard is always a valuable budget skill for units and only available at 5*, but Titania has access to it and she comes at 4* and Atk Ploy is a pretty good budget ploy for units with high res.