Veronica

I wouldn't recommend merging Veronica unless you aim to +10 her one day. (Keep in mind that she is 5-Star exclusive, so it may be unreasonable to do that unless you spend a lot of money and/or have amazing luck.) She does her job well as a single copy, and is still plenty strong.

I think that it's best to save her so you could fodder Close Guard 3; it is a really good, high-scoring C-Skill that you can give to anyone that you plan to use for Arena. Otherwise, giving Windfire Balm+ (unique to her) or Wrathful Staff 3 (though, Genny also has this skill) to another Healer that you want to invest in is a solid option.

You can merge her if you like Veronica.
Or fodder Close Guard 3 to someone like Marth that can Support his allies with Refined Falchion.