RT-55J

RT-55J is the optional sub-boss of Sting Chameleon's stage, a big sumo wrestler robot. It guards the capsule where the First Armor Body Parts (Arm Parts in Mega Man Maverick Hunter X) upgrade can be found. In the remake Mega Man Maverick Hunter X and the Rockman X manga, it talks with a robotic voice. Its body is very resistant, and its weak point is its head. RT-55J attacks by jumping over the player and trying to catch him with its extendable claws, ramming the player in the wall if successful. If its claw hits a wall, sometimes it will pull itself near the wall to hit the player in the way. RT-55J also attacks with punches in Mega Man Maverick Hunter X.

RT-55J also appeared as a boss in Rockman Xover.

Strategy
RT-55J will try to jump on X, and, if in range, he will shoot a claw out at X to try and grab him to slam him into a wall. If X has the Buster upgrade, keep charging to full level. If not, don't waste time charging, just shoot him rapidly. A good strategy is to wall-jump up a wall while he's jumping at X, then jump over and in front of him. He will try to jump on X or try to grab him with one of his claws. If he uses his claw, dash out of range and rapidly attack him. It will cause much more damage than a single charged shot. But even better is charge up while wall jumping, and do the aforementioned strategy, and when he sents out his claw, dash out of the way, release the charged shot and hit him with a flurry of normal shots.

Another easy strategy is to stay on the far edge of the screen and wait for RT-55J to jump. When he does, dash under him (assuming you have Leg Part, from Chill Penguin's stage). Take a charged shot at his head and repeat. As long as you are far enough across the screen, he will always jump instead of using his claw.

Trivia

 * Interestingly, RT-55J bears a striking resemblance to Auto, a robot Dr. Light made, both in the color scheme and appearance. Given the fact this robot guards one of Light's capsules, it is unknown if this was a coincidence or an actual reference to Auto.
 * On a related note, the first part of its name, RT, contains the same first and last letters of "Rightot", the original Japanese name of Auto.
 * There is a glitch that can be performed when fighting RT-55J. If you manage to be grasped by it the moment it is destroyed, you will be frozen in the air, as the game still registers that he is grabbing you. The player then is forced to reset after this.