Rare Akuma is an edit of Akuma created by Phantom.of.the.Server (P.o.t.S) which has a standard basic move layout, but a rather unexpectedly odd twist in his detailed moveset, as well as a huge variety of other moves he can execute.
Being a joke character, Rare Akuma is debatably the most powerful Akuma character ever created, on par with characters like Shin Shaitan, Hyper Akuma or Mega Akuma, especially when computer-controlled, he can quickly defeat his opponents in less than 20 seconds. Rare Akuma creates dozens of skulls and smaller clones of himself in seconds to bash his foes. Notably, he sometimes goes to his foe and uses an automatic K.O., 999-hit Shun Goku Satsu. After that, he will either stomp his foe while being enlarged, cause his foe to explode due to an extreme drop, or send his foe to an unknown world with a Statue of Liberty-like structure of himself, where his foe dies due to shock Planet of the Apes style. He can also stop time and teleport.
These aren't his only tools. He can also bring out a 'Katamari' of dead characters he previously fought. In the newest Rare Akuma, if the opponent is defeated with the Katamari, he/she is attached to the Katamari and Rare Akuma throws it to the sky. Rare Akuma also brings out a moon or the Death Star and hurricane kick on top to crush his enemies (parodying Dio Brando'sRoad Roller Da!). Rare Akuma is immune to projectile and magic attacks as of the latest version.
When Rare Akuma wins a match, he either summons a Royal Rainbow out from him with the Final Fantasy victory fanfare playing, performs an earth-shattering punch and the screen pans out to the whole planet to show just how far the force of the punch penetrated (similar to the Kirby Super Star minigame Megaton Punch), becomes chased by other characters (who then become chased by a Rare Akuma army), or breaks down into a large amount of miniature Rare Akumas that run away as they land on the ground. When Rare Akuma enters a match, he comes from a PokeBall, gets dropped by body parts, have Ryu and Ken fuse together to form him, laying down as normal Akuma while smoking marijuana, before turning into Rare Akuma, and so on. In terms of voicepacks, there's two available: one by ShadowWolf, and the other by The_None.
Rare Akuma is an extremely tough opponent to a human or a CPU player. He can easily resist most attacks, even being immune by F1 and F2. Some players gave out tips for defeating Rare Akuma, but these tips may vary by version. Rare Akuma is almost impossible to defeat with normal characters, characters with Super Armor fare a chance against him. One way to instantly K.O. Rare Akuma is with the use of a Reversal as Rare Akuma has no protection against them. A much stronger version of Rare Akuma exists called Giji Rare Akuma, this edit is much harder to kill and has much higher defense then the original.