Martha or Merlin

Martha is a buster oriented SR ruler, she is meant to be a damage dealer and is rather unique in that aspect. She also has decent survivabilty(nothing compared to Cu or Herc but hey, her heal at max lvl is 3000 hp). I don’t really consider her the best at her role as there are plenty of damage dealers in the game who are better, I will give it to her that she does have the unique advantage of being a ruler though. If she’s a character who you like, I do recommend to spill a few rolls on her, but if you’re looking at her for gameplay, I don’t really consider her worth it.
Merlin is someone to definitely save for in terms of gameplay, he is top tier support for buster teams. However, if you don’t want to save for him don’t bother, you’ll have half of your friend list with him anyway.
Hope this helps.

Merlin is superior in every way except solo damage output(and maybe even then with putting his crit buff on himself and creating stars with his NP) while Martha is a 4* and thus easier to get.

Not to say Martha is bad it's just when you compare anyone to Merlin they end up looking quite weak.