Although the two didn’t exactly trust each other, there wasn’t any conflict of interest, and they were both the same type of person. Garen had his pride as a member of the strong, and wouldn’t allow himself to find a weakling as a companion. He needed someone who could stand independently, a strong fighter who would not give up easily regardless of the dangers faced. He needed someone who could help him when needed, and not a weakling who would constantly need his help for every little thing.
Garen didn’t have much of an advantage here on this world. Compared to those prodigies, he only had his potential points to possibly bring him to the peak. But he had his pride, and the strong would remain strong however much their environments changed. It was just a matter of time accumulation.
Similarly, Angel had the same opinion, someone of the same level who could defeat her in direct confrontation, and the same type of person as she was. That made her acknowledge Garen all the more.