Have to agree with both Jack and Eira.
Just because some other GM let him play this character DOES NOT mean you have to... Simply tell him this is YOUR game and not the other GM's and he has to make up a new character.
If he is awakened, then keep in mind that his total Magic Rating is limited to (Essence + Initiation rank) ROUNDED DOWN. So if he has not initiated and has 5.00 essence worth of bioware/cyberware installed his effective magic rating is 1, which serves as the thresh hold for when Drain become Physical! This character COULD literally blow his own brains out by casting a high powered spell.... And considering the Max power of a spell he casts is limited to Magic Rating x2... Most spells he casts are going to be easily resisted.
Other then that, just don't "play his game".... Meaning, design you runs for the other 3 characters in mind and just go from there. Yes this character will out shine the others some times, but no character can be good at all aspects of the game. (remember, Combat is only 1 part of the game..... And players who think they can just blast their way past every problem is in for a surprise as Corps can literally send unlimited numbers of drones, spirits, critters and security guards ALL DAY LONG at the idiots that just try to shoot their way past problems... And that doesn't include the assets of the Police and even the Military that can be called in, in extreme situations (like when a bunch of "Armed Terrorists" are shooting everyone in sight!)
It is possible to mix high powered and high karma characters with relatively new characters IF the GM takes the time to flesh out all aspects of his game and applies some World Consistancies to it. Trust me.
For example my Old School group has been running the same characters for almost 30 years now (updated through the edition changes, of course). My Mage as 4700+ Karma, our Adept has 5200+ karma, our Rigger has 4400 Karma, the Sammy has 4900 karma, and our decker/technomancer has 250 karma.... Yet our decker/technomancer can keep pace with the others just fine in most regards.
One of the biggest problems I have seen over the years in shadowrun done by GMs is what I call "Enemy Creep".... and this is usually because they are more familiar with other game systems that use a "level" system. (like DnD). IN DnD, as the characters gain levels, they fight harder and harder monsters. At first level they usually fight Kobalds and goblins and Orcs. Then as they gain a couple of levels they start fighting Orcs, hobgoblins and Ogres..... then when they hit level 10 to 12 they are fighting Giants, dragons and deamons.... So they take this thought process and try to apply it to Shadowrun.... usually in the form of additional dice and higher health, armor and weapons.....
Meaning when the characters are starting out and fight a group of gangers, the gangers are armed with pistols, armored jackets and have a dice pool of 4 to 7. Then after the party has a few dozen karma... those same gangers have submachineguns, heavy armor and dice pools in the 7 to 12 range... Then when the party hits a couple hundred karma the gangers are now packing Assault Cannons, light security armor and have dice pools of 10 to 20!
This is (in my opinion) a Fallacy. A ganger is a ganger is a ganger. Forever and always. That is not to say there won't be variation among the gangers, but they are not improving in leaps and bounds on par with the players... they are still bound by the setting and the "reality" of their situation.... Take my above group for example, we STILL deal with gangers (in fact we just finished a mission dealing with some gangers trying to squeeze an old contact buddy of the Samurai) And yes, we MASSIVELY out class them... For example they were rolling 8 dice to shoot with 16 dice to resist (modified more by AP), while our Samurai rolls 26 dice for his LIGHT pistol!
Our problem wasn't the combat as both me and the adept can auto-success more then they can Edge Resist... Our problem was getting to the hostage and keeping him safe from the gangers, while dealing with socio-political fallout of the situation and not letting it degenerate into a blood bath that spilled out into the general public... And that takes more then just "shooting them in the face".... We were never in any real danger of taking a single point of damage. (well.. I did.. but that was self induced.... once by drain, and once because I glitched a falling roll.... but.. Meh, I was never in any real danger of dying and 4 stun/2 physical is nothing to really worry about).
No, the issue you are going to have is the resources this single character bring tot he table and the imbalance and personal jealousy the other players will have when they see this other character... (This isn't an issue at our table asn the decker/technomancer choose to retire their high karma character and KNEW they would be starting much lower then everyone else. But she is ROLEplayer.. and not a ROLLplayer... for her, the fun is in the story and the character, not the dice she gets to throw)
