According to Buddhism, there are three central facts of life that must be faced. 1) anicca, which is impermanence, 2) anatta, which is that there is no permanent identity such as the soul or self, and 3) dukkha, which is translated suffering. Although these three aspects of reality are identified by different terms there are, as it were, three sides of the same thing.
As best as I have been able to tell, Buddhists hold a modified doctrine of reincarnation. There is samsara, the cycle of birth and rebirth, but that which goes to the next incarnation is not the identical person that lived before.
The "official" teaching of the Buddha, (unless he secretly gave Mahayana doctrine to some disciples) is that devotion to anyone is useless in the pursuit of Nirvana. Either the being you wish to devote yourself to has achieved Nirvana, in which case the relevant being has no more desires, and hence no desire to help you, or else that being hasn't made it yet, in which case that being doesn't merit devotion.
Mahayana doctrine modifies this position by saying that some beings of great power have chosen not to enter Nirvana, but rather delay their entry in to nirvana in order to help everyone else make it. In so doing, they allow themselves to go through Samsara (being continually reborn), so that others can be helped.
The question I have is whether Mahayanists, in accepting this understanding of Bodhisattvas, also have to modify their understanding of anatta as well. A Bodhisattva like Guanyin, for example, is presumably the same being each time she reincarnates. Has this issue ever been raised by Buddhist scholars?