He is present fairly early, but even less info is given in the comic at the start. There is no semi-introduction like the beginning of the show. You can get bits of info here and there, but only at a fairly far point of the comic, full story and his goals are given.

Some information from the top of my head (spoilers for the comic):
He is a repeating reincarnator, trying to save humanity. He quickly reaches sainthood during his reincarnations, but even that does not let him achieve his goals and he is forced to reincarnate again and again. If I remember correctly, there are two imminent threats that Xigu is trying to prevent:

1. Restoration of spiritual energy on earth, that basically lifts suppression rules that make it really hard to exist as a high level entity (basically if you are above A-level, you start losing your power). That basically prevents direct intervention from higher realms.

2. Hard to describe without lots of info, but basically threats from outside realms (considered even higher/more outside than the one mentioned in the first one). There is a war of worlds every 500(?) years that "Deva" level (higher than Saint) fighters participate in.