The Shifter is the second of the Occupations in the core rulebook derived from the archetype of the Western High Ceremonial Magician. This Occupation, in contrast to the Ley-Line Walker, focuses its skillset and magic knowledge upon the practice of ritual magic- specifically, on rituals that cross barriers of time and space to allow communication and travel across those barriers. The closest synonym, in Palladium's offerings, to this Occupation is that of the Summoner from Palladium's fantasy game; this means that the Shifter is at its best when played by an individual that comprehends how dependent this Occupation is upon preparation, planning, and attention to detail.
The Shifter's emphasis on ritual magic requires that a Shifter pay attention to the geography of the area, including its inhabitants. Does it have a ley-line? Does it have a nexus? Can he reliably access that power, if present? With whom, if any, does he compete with to acquire that power? Why is he so concerned? Because he cannot possibly work his ritual magic using only his own cultivated reserves of mana. He has to find and use mana sources outside of himself, or his rituals cannot work; he won't have the power necessary to succeed.
The Shifter also needs to keep careful and meticulous records of time, due to the effects that particular moments and occurrences have on their access to external power sources. The Shifter, if he suffers certain deficits of character, may also become proficient--if not expert--at medicine for purposes of practicing animal and human sacrifice; this too is a proven, and therefore known, way to acquire mana external to one's own reserves. Combined, all of this means that a competent Shifter will have a significant bias towards being meticulous, detail-oriented, practitioner of magic who relies on planning ahead and preparing accordingly to succeed.
The Shifter cannot avoid being charismatic, strong-willed, and intelligent if he wishes to succeed; the best of them, contrary to expectations, will also be healthy and strong in body due to the connection between bodily and mental health. He regularly traffics with entities comprehensible (variously so) to him, and is capable of striking bargains with some and compelling the obedience of others. Contrary to expectations, this requires that the Shifter has a command over interpersonal skills that one would expect from someone trained to command subordinates and parley with equals- someone trained to lead men. (It is not accidental that mythological examples have such backgrounds.) Because he needs to prepare, he does not travel light or often; he's the sort to think in terms of expeditions when departing from well-traveled roads (literal and otherwise) and act accordingly. He is likely to be found at one of his sanctums, as his emphasis in magic requires prepared spaces and other infrastructure; while fully capable of casting spells, what he knows is far fewer in quantity than a Ley-Line Walker and often focused on utilitarian needs. Veteran Shifters acquire an entourage, a warband, of henchmen and servants; some are ordinary folk, some supernatural, all bound to the Shifter and acknowledge him as leader- if not master.
While many Shifters do reveal themselves deficient in character over time, and revel in evil accordingly, not all of them do. Far more likely is that the Shifter simply becomes obsessed with the potential of his practice and loses sight of the reason for why he took up this Occupation originally; in other words, you're far less likely to find Faust and far more likely to find Radagast- someone who takes up residence in a particular place of significance (if not power) and becomes increasingly immersed in his particular pursuits. (If left alone, he is practically harmless.) Dealt with in the proper manner, one can parley profitably with such a Shifter; do it wrong, and you're in for a world of hurt- if he's merciful.
Using a Shifter for a NPC is a good choice for someone who either acts as a mentor and captain for a group, or for a group's opposition. Their powers over cross-dimensional barriers makes then particularly good for adventures and campaigns focused on making the most of what RIFTS has to offer; in this respect, they are equal in potential to Temporal Raiders and their playable Occupations (Wizards and Warriors). They make for great NPCs that a Game Master can do some solid world-building, in the local and regional sense, around; if you need someone of note who's big on otherworldly stuff, a Shifter is your go-to option.
So, if you want to think ahead, be prepared, and otherwise pretend that you're a Time Lord then this is the Occupation for you.