His allies could include Hermione and Ron as always. Since they are his best friends, they would support him in his apprenticeship. Hermione might even help him study and research with Flitwick. Neville could also be an ally as he is also interested in learning more magic. As for rivals, Draco might be jealous of Harry's new position and try to undermine him. Some of the Slytherins who don't like Harry might also be his rivals in this scenario.
Allies could be the other professors at Hogwarts aside from Flitwick. For instance, McGonagall might encourage Harry in his new role. Hagrid could also be an ally, providing moral support. When it comes to rivals, there could be some older students who feel that they should have been chosen as Flitwick's apprentice instead of Harry. And there might be outside forces, like some dark wizards who hear about Harry's apprenticeship and see him as a future obstacle to their plans, so they try to make him their rival.
Potential allies could be the members of the Dumbledore's Army. They would see Harry's apprenticeship as an opportunity for the whole group to learn more. Luna could be an interesting ally too, with her unique perspective on magic. Regarding rivals, some students who are ambitious about getting special treatment from Flitwick might see Harry as a threat. Also, any Death Eater sympathizers within the school could target Harry as they don't want him to gain more power through this apprenticeship.