Star Trek: Discovery's Red Angel is clearly from an alternate timeline that is similar, but not identical, to the Prime Star Trek timeline. The identity of the Red Angel is the abiding mystery of Star Trek: Discovery season 2, and the series is doing a good job of building up the sense of intrigue.
The Red Angel's mysterious plans are at the heart of Star Trek: Discovery. It appears to have chosen Spock and Burnham as its agents, intervening to save Burnham's life when she was just a child. Its goal is also becoming increasingly clear: it's attempting to avert an act of cosmic genocide, working against a rogue artificial intelligence that threatens all organic life in the universe. The being's secrets are sure to be revealed soon, since next week's episode is called "The Red Angel," which presumably means the Discovery crew is finally going to unmask the creature. That makes this the perfect time to take stock of what is now a near-certainty: that the Red Angel originates from an alternate future timeline.
The Red Angel has been appearing throughout history. Its first recorded appearance was in 2053, when it intervened in World War III to save a group of soldiers and civilians who were huddled in a church in Richmond, Indiana. More recently, it visited Spock during his childhood and ensured he saved his sister from being killed by a savage Vulcan creature. It's only recently that the Red Angel has chosen to make itself known on a cosmic scale, causing significant disruption on Saru's homeworld of Kaminar and preventing the Ba'ul from committing an act of genocide. That last appearance left a rift in time and space, clearly indicating that the Red Angel travels through time.
Fitting with this, the Red Angel has intimate knowledge of the timeline. When it appeared to the young Spock, it projected an illusion into his mind that showed where his runaway sister was - and that she was about to be killed. Spock was able to tell his parents where Burnham was, and they got there in time to save her life. More recently, the Red Angel revealed the seven lights to Spock before they flared into existence, again indicating that it has knowledge of what is yet to happen. Most disturbingly, it has also given Spock visions of a universal apocalypse, of the end of all life in the cosmos. It's reasonable to assume that, just as Spock was able to prevent his sister's death, the Red Angel hopes the crew of Discovery can avert this horrific future.
Saru's enhanced vision revealed that the Red Angel is a human, wearing an advanced suit beyond anything the Federation has ever seen. When Spock briefly managed to establish a mind meld with the Red Angel, he sensed loneliness and desperation. Putting all these pieces together, then, it looks as though the Red Angel is the sole survivor of a cosmic apocalypse that saw all other organic life in the universe destroyed. It has traveled to the past in order to rewrite the timeline, subtly at first but now directly as the key moment draws near. Next week, presumably it will at last reveal itself to the crew of Discovery.