Warning: contains spoilers for Immortal Hulk #45
Just because every Hulk is an important part of Bruce Banner's psyche, doesn't mean they always get along. While the most famous conflict is of course between Banner himself and the standard, child-like Hulk, other Hulks have frequently had problems with Banner, and none more so than Joe Fixit. While Joe might be the darkest Hulk, he typically isn't the strongest, but all that is set to change as he puts aside his differences with the other Hulks.
Though usually treated as more of a side character, Joe Fixit has risen to some prominence in the last few issues of Immortal Hulk from writer Al Ewing and artist Joe Bennet. After Devil Hulk and the Green Scar were killed and Bruce Banner was abducted by the Leader, only two Hulks remained. Joe and the child-like Hulk typically don't have the best relationship, as Joe's selfishness usually gives him little patience for the Hulk's childishness. However, that changes when the two are forced to rely on each other as they go on the run from government agencies that view them as nothing but a dangerous threat. All that came to a head when the villainous U-Foes came and roasted the Hulk, sending Joe and Hulk straight to the Below Place.
Now, in Immortal Hulk #45, things are more dire than ever for Joe and Hulk. The Hulk's archnemesis, the Leader, fused with the One Below All to create an eldritch nightmare that rules over the Below Place. Hulk and Joe initially have hope that they'll be able to escape as one of the Green Doors that allow all gamma mutates to resurrect appears, offering them a way back to the mortal world. They try to take it but are interrupted by the Leader, who sends wave after wave of monster to destroy them. They manage to rip Hulk in half, which manages to spark something in Joe. Before both are taken by the Leader's creatures, Joe transforms into the Red Hulk and saves them both. He regenerates back to life, gray as usual, but now crackling with red energy.
One thing that makes this moment stand out in Hulk history is that it's the first time any of Bruce Banner's Hulks have ever turned red. Previously, red was the color of gamma mutates such as the Red Hulk Thunderbolt Ross and his daughter, Betty. While the Red Hulk's are typically on par with their emerald counterparts, Joe's red form displays a level of strength that the Hulk rarely achieves.
It's notable that, when Joe regenerates in front of the U-Foes, that he specifically uses the pronoun "our" to refer to himself. This suggests that this new Red/Gray Hulk might be an entirely new persona. It's possible that there is also more than a little bit of the tragic Devil Hulk in this new persona, as his threatening dialogue and menacing grin are more than reminiscent of the fallen Hulk. Regardless of who or what he combined with, this new Hulk persona is now the strongest of the strong.