Sander Cohen
Sander Cohen runs Fort Frolic with a hard dick after having locked the area down to prevent its remaining inhabitants from escaping. As with the Medical Pavilion's Dr. Steinman, he is insane, driven so partly by plasmids and out of frustration for the lack of appreciation for his work (an audio diary reveals that he regrets his decision to move to Rapture). He was given the key to operate the level by Andrew Ryan.
Before Rapture decended into chaos, Cohen was still an important factor in Fort Frolic, owning a gallery of art, as well as producing several shows, one of which, name "Patrick and Moira", was obviously an inspiration for Fontaine and his alter ego. Cohen was the most notable enemy of another singer/actress, Anna Culpepper, called "Ryans songbird". He used his contacts with Ryan, who specifically chose Cohen to join him in Rapture, to "Put the bump" on her, as said by Security Cheif Sullivan, who had to do the dirty work. Her fate is unknown, but with the competition gone, Cohen experienced a rare era of extravagance before his downfall into insanity.
During the time between new years eve 1959, and the time that Jack arrives, Cohen apparently has upwards of 4 desciples, although he may have had more. The first was apparently either Silas Cobb, Martin Finnigan, or Hector Rodriguez. Cobb, a member of Cohen's inner circle before '60, probobly because he sold Cohen's records in his Store. He practicaly worshiped the "Musical Genious" that he saw Cohen as. That is, until Cohen deemed him "Unworthy" and shut him in his own record store. When Jack arives, Cohen sends him to kill him an photograph him, to complete his masterpiece. Before he is killed by Jack, he reveals in a diary that he thought he was a genous because, as he said, "You paid the rent, you ancient Hack!" He is killed and photographed for the masterpiece.
Hector Rodriguez was most likely a man who spoke out against Cohen and his lot during the days of sanity. However, he was likely given a chance to repent by Cohen as his desciple. He remained his ideal that art was "all Grift", and like the rest, was vengefully put in the freezer of poseidon plaza. He frequented the Stip Club, Eve's Garden, and when the plasmids drove him mad, he aparently created the delusion that the place is still up and running, although with slightly worse service. Jack finds him there when he is promted to photograph his corpse for Cohen's masterpiece.
Martin Finnigan was a shady fellow, who apparently had no problem using drastic measures. After being tossed in the freezer for being unworthy, he dominates the others, making himself a plasmid cocktail, and calling himself "the Iceman". He freezes his victims and puts them in dramatic poses. He attempts to do so with Jack as well, but ends up dead and photographed on Cohen's masterpiece.
Finaly, the last certain Desciple was a young man named Kyle Fitzpatrick. Cohen chains him up to a Grand Piano in Fleet Hall, the theater in Fort Frolic. Cohen commands him to play his score perfectly, and when Fitzpatrick refuses to play anymore, Cohen detonates the TNT attatched to the Piano. As it turns out, the masterpiece was apparently written as a requiem, for Cohen was readily prepared for his failure and had Jack photograph hi corpse, as if it were all part of his plan.
He has the player (Jack) hunt down and kill the 4 afformentioned men and take pictures of their bodies to finish his "masterpiece". After the player completes his Quadtych, as the work is known, he will reveal himself, amongst a host of spotlights and confetti. When he talks to you, he will congratulate you on your work, and then reward you by opening one of the glass cases nearby the stage. If you are satisfied with your new plasmid, you can go on your merry way, and you do not have to kill Sander Cohen. If the player so chooses to let Cohen live, his fate remains unclear (regardless of ending). Ryan's words later on in Hephaestus and that Cohen remains stood there appreciating his Quadtych suggest that he remains in Rapture till his death.
Xbox 360 Specific Where you choose to kill Cohen affects which achievements you can obtain. If you let Cohen live in Fort Frolic, you will be able to gain access to a secret area in his living quarters in Olympus Heights later in the game. Entering the second floor of his apartment will give you an achievement and an otherwise unattainable Power to the People vending machine. To gain access, simply disrupt the two dancing Houdini Splicers in his apartment; Cohen will then come down to kill you. Obtain the second achievement by taking a photo of his corpse.
If you want to get back at this psychopathic thespian promptly, you may attack him in Fort Frolic. He is relatively strong and uses a similar attack pattern to a Houdini Splicer except with more health, and a much larger teleportation range. When you kill him, search his body to find some EVE hypos and a key, which will grant you access to the second glass case containing the muse box, for some money and items.
To receive both achievements quickly, simply save in Fort Frolic after Cohen has revealed himself, but before you attack him. You may then initiate a fight, once he is dead, take his photo to unlock the achievement and then reload your save to before you attacked him. Then allow him to live, and leave the level without attacking him.
Note that if you choose to wait to kill him in Olympus Heights, you may return to Fort Frolic after killing him, set the Quadtych on fire, and he will reappear in a rage. This is either a "known issue" or an odd story dynamic; if Cohen is related to Ryan, he could use a Vita-Chamber--he can certainly use the bathysphere, as there is no other known way to Olympus Heights from Fort Frolic, and he beats you there--and it would explain his sudden reappearance.
PC Specific Killing Cohen in Fort Frolic is sadly pointless, because there are no special achievements in the PC version and the treasure inside the muse box is pathetic. Although, if you do not attack Cohen in Fort Frolic, you will be able to gain access to a secret area in his living quarters later on in the game in Olympus Heights which in turn will give you access to an otherwise unattainable Power to the People vending machine. To gain entry to the second floor of Cohen's apartment, simply disrupt the two dancing Houdini Splicers in his apartment, Cohen will then come down to kill you.
There is a notable glitch in the PC version; if you kill Cohen in Olympus Heights you may still salvage the key to the Muse box back in Fort Frolic. However, if you go back to Fort Frolic to unlock the muse box, Cohen will be alive and instantly aggressive towards you.