She's pretty stacked (lol)
A pre-promote unit with high base stats, but a lower internal Level (15 + 0) means she gains EXP like crazy. She's got high movement, hits hard, is fast, can handle just about any enemy with Axes + Dual Club, can go Wyvern Lord and dominate even harder (she can flat out solo chapter 19 with a beast killer). Ninjas are the enemy™ and axes beat them so she's extremely effective in maps where they're a pain. Chapter 10 would
have been a hell without her. Her only flaw is her sketchy HP growth, and a single Seraph Robe + constant HP tonic use fixes that forever. If you really want to see her go nuts, pair her off with Keaton, who gives her enough Strength and Speed to ORKO most enemies (and the resulting Velouria is pretty OP too).
Re: Felicia
Felicia and Jakob are special; instead of hitting the level 20 cap, they cap at level 40 instead. This means they can be in a promoted class, but gain normal EXP which is pretty goddamn incredible. But their combat falls off due to their poor growths, so i'd strongly recommend keeping her permanently attached to Corrin as a pair up option.