In Star Trek Beyond, Krall (and his accomplices) maintained his longevity through an unexplained energy transference technology that drained other living creatures. Krall's appearance changed during and after this procedure changed, reflecting distinctive characteristics of his victims.
Jaylah, played by Sofia Boutella in STB, had minor facial prosthetics. It was limited to a smooth ridge extending up from her orbicularis oculi and gently hooking toward her temple. The look is so subtle, a casual viewer might miss the fact that she has a prosthetic as part of her costume.
Watching STB, I started to think that Jaylah looked like Krall. But then, realizing she knew her father and saw him die (or at least was aware of his death by Manas) I started, instead, to think Kraal must look like Jaylah.
Krall, played by Idris Elba in STB, had a far more elaborate prosthetic. It included almost every aspect of his head. His make up was certainly enhanced to make him more menacing and scary. But in the final scenes of STB it became more clear his appearance was most like those he consumed.
Why does Jaylah look so much like Kraal?
Here's what I kinda of think:
Krall's appearance may be an amalgam of species, but specifically punctuated by Jaylah's fallen comrades who contributed to his longevity and appearance. It is reasonable to infer she is on the planet Altamid from a fallen vessel whose crew compliment may have been large and "lasted" Krall years. More interesting, it is possible that (in a way) when Jaylah sees Krall and Manas she clearly sees her father's echo in their appearance. If any of this is true, and it may be, then her character's courage to face and engage the enemy is remarkably notable.