Who was that speedster that had Supergirl's powers, in the Supergirl/Flash crossover?
Was he really a good guy or will he become Flash's next opponent?
No "speedsters" visit Earth-1 during the crossover episode Duet (S03E17).
Supergirl is brought to Earth-1 by two people:
J'onn J'onzz, AKA: The Martian Manhunter - a shapeshifting Martian who has enhanced strength, telepathy, and flight capabilities.
Mon-El, the Daxamite, who has powers largely identical to Supergirl herself.
Both characters are on Team-Supergirl, and are highly unlikely to ever become villains on The Flash.
The Music Meister also made an appearance during this episode. He also does not appear to be an inherent speedster, only demonstrating speed abilities while drawing energy from Barry.
At the end of the episode,
Music Meister claims that he never meant to hurt the heroes - and that he was trying to help them in his own way.
Even Darren Criss has said that he does not see Music Meister as a villain. From his interview with EW:
A lot of people have been asking me, “What villain are you playing?” I’m like, “He’s not really a villain, he’s more just kind of an a–hole.” They let me run loose a little bit as far as being silly and goofy with the character, and I push it a little too far and they’re like, “Make sure to dial back a little bit,” because I’m trying to keep it in the tone of the world. But he’s definitely a goofy — a meister.
While there have been a few tiny references to Music Meister already, there has, to date, been no news of a recurring role for the character to become a villain or foil to the Flash.