For more information about Charlie, please see his Street Fighter Alpha entry.
X-Men vs. Street Fighter sees the first appearance of Guile's U.S. Air Force commander outside of the Street Fighter Alpha series. Charlie is pulled into the Marvel universe along with several other Alpha fighters. While his Sonic Boom is not augmented, his Somersault Shell kick is given an extra release of power at the end that knocks his opponents high into the air even after the initial strike. He gains an additional air move that is like the reverse of the Somersault Shell. The Moonsault Shell like an air to ground somersault kick that attacks opponents beneath Charlie. He gets no new Hyper Combos, but the ones he has have been enhanced a bit. Sonic Break launches a flurry of Sonic Booms, and not just one at a time. If you don't tap any buttons when performing the move, he will send out at least 9, but if you tap punch, you can send out an addition 6 for a total of 15. His Somersault Justice is enhanced just like his Somersault Shell, and his Crossfire Blitz is simply more exciting to look at.
The Marvel vs. Capcom series provided a twist on Charlie's fate. In his ending for X-Men vs. Street Fighter, Charlie is captured by Bison, who performs terrible experiments on him. Charlie's fate would be left up in the air until Marvel Super Heroes vs. Street Fighter, where a secret character by the name of Shadow appeared. Shadow was a palette swap of Charlie, with nearly his entire body blacked out, and a white eye shining behind his glasses. He has all of Charlie's moves, but his attacks will set his opponent alight with blue Psycho-Power flames. His supers also have a brief pause where his eye will gleam before the move is executed (giving an opponent enough time to block or avoid the attack). Shadow later appears in Marvel vs. Capcom as a secret helper character. However, by Marvel vs. Capcom 2, Shadow seems to have reverted back to his original form, but by unknown means. In addition, the ending for Chun Li also hinted that Bison intended to alter her in the same way, and another hidden character, called Shadow Lady, was a Shadow-altered Chun-Li. Shadow Lady's ending in the game also showed a similar fate befalling Jin Saotome.
Moves
X-Men vs. Street Fighter
X-Men vs. Street Fighter/Charlie
Marvel Super Heroes vs. Street Fighter
Marvel Super Heroes vs. Street Fighter/Shadow