REIKO

Origin: Unknown
Species: Human
Affiliation: Worked for Shinnok

Mortal Kombat Gold: Long, long ago, Reiko was the general of Shinnok's
unholy army. But it was thought that he was killed in battle. Now that
Shinnok was spreading his wrath once more, Reiko made a mysterious
reappearance. He fought with his master, but was defeated with the rest
of Shinnok's faction.

Ill edit with the rest of the answers.
Real short answer of the top of my head, about reptile, he worked for shinok in mk 4 so he can have the opportunity to return to his homeland.

Most of the endings are what if type things. Sometimes what is shown could've happened but didn't. In some endings Raiden gets killed it's all relative.

Q: who is reiko?
A: he was a servant of sorts i think for shao kahn, but his ending shows that he is really shao kahn.

Q: why sonia says that the black dragon died with kano?
A: sonya thought that the black dragon was dead as kano is soppused to be the final member but she founded out that kano was still alive in mk:da, like mavado.

Q: why sonya is alive in mk5?
A: jarek sucks so his ending wasnt true so sonya didnt die

those are my suspision i guess, or think, i dont know, hope this helps