The Mortal Kombat franchise is filled with obscure, secret characters, many of which essentially look like color-swapped versions of existing characters. There's no better example of this than the various multi-colored ninjas in the series. Though they may look like icons like Sub-Zero and Scorpion, these are entirely separately characters with vastly different elemental powers.
One incredibly esoteric palette swap character is Hydro. Hydro looks a lot like Sub-Zero combined with Scorpion, but despite this interesting design, he's remained a relatively unknown part of the franchise since debuting in 1994's Mortal Kokmbat: Blood and Thunder #1, by Charles Marshall, Patrick Rolo, Bobby Rae and Dave Lanphear. This insignificance has kept him from ever being a playable character in the games, which is unlikely to change anytime soon.
Hydro of the Lin Kuei
Hydro's debuted in Malibu Comics' short-lived line of Mortal Kombat comics in the '90s. He was a Lin Kuei warrior from Earthrealm, and a close friend of fellow warrior Sub-Zero, Bi Han. His costume was very similar to Sub-Zero's, although it was a darker shade of blue and lacked the black in Sub-Zero's costume. Fittingly, his powers were also very similar to Sub-Zero's, though his element was water instead of ice. Despite this, his water could be turned cold instantly freeze an opponent like Sub-Zero's powers, making Bi Han a bit redundant.
Perhaps for this exact reason, he only appeared in a few issues of the series. In Mortal Kombat: Blood and Thunder #4, he's killed by Scorpion, who harbored an intense grudge against the original Sub-Zero. This was part of Scorpion's plan to eliminate all of Bi Han's allies before killing him as well. This murder is rather ironic, given that Scorpion's element of fire should logically put him at a disadvantage against Hydro. His ability to control water also makes him similar to later game character Rain, although the latter has a far different, admittedly tongue-in-cheek color scheme.
Hydro In Mortal Kombat: Legacy
For the most part, Hydro has languished in obscurity since then, and he's failed to show up in the games as a playable character. His next appearance was in the 2011 live-action web series Mortal Kombat: Legacy, where he'd be given a much different storyline. There, he was an early victim of the Lin Kuei's controversial cybernetic ninja program, retaining his Sub-Zero-esque blue color scheme. He's used to test the abilities of newer cyborg models Cyrax and Sektor, and shares their ability to charge his attacks with electricity. Unfortunately for him, they were far more advanced than the outdated model of ninja and quickly overwhelm him. Sektor holds Hydro back long enough for Cyrax to perform a Fatality, quickly ending Hydro's story.
His friendship in the comics with Sub-Zero isn't mentioned, which makes his heavy resemblance to him somewhat questionable. He's also established as having been made into a cyborg at 50 years old, making him the oldest of the Lin Kuei ninja seen in the series. So far, his only "appearance" in the video games has been a cameo in Mortal Kombat 11, and even that wasn't much. He doesn't appear physically in the game, but is a mere modifier for the "Towers of Time" mode. There, he summons a debilitating wave of water to swallow up an opponent, making them lose control.
His similarities to the aforementioned Rain are the likely reason for the developers' ambivalence to introduce him as a playable character. With Rain currently being playable in Mortal Kombat 11, however, NetherRealm Studios may switch things up and make Hydro one of the fighters in the sequel.