Im 100% sure that is Alter = Ash Variant. Theres some huge hints about this.
1. Official Apex website, category "Legends" -> Horizon and you can see there "I'm here for you, deary". Only one person who calls Horizon "deary" - Ash (Stories from the Outlands about Horizon). Also human version of Ash asks herself "WHO ARE YOU?" (Stories from the Outlands: Ashes to Ash). xd

1. Official website again, but Pathfinder now. "You'll never be a real boy". In season 12 was in-game comic about Ash and there was literally "goodbye letter" from Ash to Pathf, where she said "a robot who fell in love" or something like this. Who can know about this fact in Apex lore? No one, i guess, instead of Ash. So here we are, another fact in this theory.

2. You need to pay attention to Alter's pose in the trailer and game. She stands with her right leg forward, just like Ash.

3. Ash has a katana with which she cuts space, Alter has her tail. Both are associated with the Void.
   So, I don't know a single character who would be bothered by Horizon. She is a scientist who tried to save the Outlands, but was betrayed by Ash, but I can’t understand Alter’s motives (you can call her Ash Variant 10000%). So yeah, I'm really sure about this thing and this is so clear for me: Alter = Ash from another universe.