The Time Guardian is a member of the Guardians of Time a group of nigh-omnipotent entities which personify concepts of the universe. The White and Black Guardians are also members of this pantheon.
The Time Guardian is the personification of Time itself and his purpose is to ensure that no paradoxes or massive changes to the timeline occur. To that end he creates fixed points in time and carefully manipulates parts of the Doctor's(and presumably other time travelers) timeline in order to ensure that he prevents fixed points in time from being changed.
During the dark times the Time Guardian had to impose many fixed points in the universe so that in the likely event of manipulation of this time period via time travel can be prevented. He compares this to being as hard as trying to solve the Skasis Paradigm using only a pocket calculator(GRD: And I Heard).
Dealings with Morbius
When the Time Guardian discovered Faction Paradox' plot to conquer the universe he imediately ordered the White Guardian to instruct Morbius to resurect the Time Lords in order to combat the Faction(GRD: Art of Time). The result was Morbius rallying the surviving Time Lords including the Doctor. With the Time Lords assembled the Time Guardian(through the White Guardian) revealed to Morbius that a planet restoration device exists. With this device Gallifrey and the Time Lord race can be restored(GRD: The Guardians: Redemption).
The Quest for the Time Guardian
Oh this, well I just picked it up. - Time Guardian The Time Guardian's humanoid form is that of a teenage human male with brown eyes and hazel coloured hair. His attire consists of a red pinstriped shirt, a gold coloured bow-tie, a white jacket, a black overcoat and a pair of azure pants, he also wears a crystal wristwatch. His multi-coloured clothing is used to signify the other Guardians of Time - the Red Guardian(Justice), the Gold Guardian(Life), the White Guardian(Order), the Black Guardian(Chaos), the Azure Guardian(Equilibrium) and the Crystal Guardian(Dreams).
What, do you want me to draw you a map?! What are you an amoeba!? - Time Guardian The Time Guardian has a very sporadic and alien personality. He sees other lifeforms(including Time Lords) as inferior and he often makes sarcastic remarks about their lack of knowlege of events to come. When he shows Morbius a vision of the future he doesn't display much sorow for the fate of the universe, but nevertheless he wants to save it. He sometimes displays a sadistic view of life and death saying that the Time Lords deserved their fate for being so overconfident.
- This is the year 2.3/We're screwed/(beat)5, ten years no wait(looks in his watch), ten days in your future and this is the day the Doctor dies.(looks at Morbius) Welcome to the end of the universe. - to Morbius.
- Right now I'm trying to save the universe from being destroyed and it realy would help if you could be more serious.