In Smackdown vs Raw 2006 Taker can play Smackdown's! season mode, which soon leads to him fighting JBL for Torrie Wilson's contract. Out of character right? Except that Calaway had been a legitimate Papa Wolf in this regard, for real, and it would be completely in character to be in an angle and free Torrie from the slimy clutches of the Texas tycoon, and most likely just tear up the contract afterwards.
The stories about Mark Calaway stopping backstage horseshit are legion, and by 2006, this was very well known on the Internet. Kane joining in would fit here- Jacobs is generally acknowledged as Taker's deputy on this, especially when they were separated by brands.
Going more on the Divas, it was said Batista was a bit of a womanizer and hit on a good few divas, whom he would try to get fired if they rejected his advances. Well when Mark learned about this, it was said he made sure to put a stop to it.
One could easily argue that connection is (in-kayfabe) part of why he lost - for one who is so closely bound to Death, being in a land so tied to the overcoming and commanding of death slowly weakened his will and ability to fight, until even his nigh-invincible power was drained away.
His Real Life mentorship of Drew McIntyre during WWE's first run shows COMPLETELY when you look at who Drew became ever since he got fired, from his fiery return to ICW in 2014 to his 2020 run as WWE Champion. Much like Undertaker, McIntyre is a physically imposing Dark Is Not EvilJack of All Stats with the promo skills to back up his ability.
Why did Undertaker arrive during the Cody Rhodes vs Roman Reigns match at WrestleMania XL instead of Stone Cold Steve Austin? Not only is it because Roman Reigns gave Undertaker his second loss at WrestleMania, but it also harkens to his debut match, where he was part of Ted DiBiase's team against Dusty Rhodes.
One could also infer that Undertaker, being all but the physical embodiment of WrestleMania itself, was sick of seeing Roman Reigns and The Bloodline desecrate the biggest show of the year for nearly half a decade straight. WrestleMania is the show that Undertaker spent his whole career sanctifying— heel or babyface, The Undertaker never cheated in any WrestleMania match, and he finally refused to let Roman and The Bloodline continue to do so. He was also likely aware that there was just about no one else who was both willing and able to stand up to (and vanquish) The Rock on Cody's behalf, so he took matters into his own hands.
If Calaway were to suffer an untimely death, how on Earth would WWE convince the world that The Undertaker really was dead?
For in-kayfabe Fridge Logic; if 'Taker is some sort of supernatural being, why in the world does he spend his time wrestling?
The same reason Dark Brain forced his worst enemies into a completely harmless dodgeball tournament, for shits and giggle.
You can also interpret his character (if you so wish) as a normal guy like the rest of the roster who uses special effects and illusions to intimidate his foes into thinking he's some mythical being in the same way Batman does.
An alternate interpretation is that he's similar to the character of Ghost Rider. He's a normal human who was possessed by a satantic entity, and the two of them share the body in a sort of symbiosis. After Johnny Blaze sold his soul to Mephisto, he still continued his career as a stunt biker despite battling dark, supernatural forces when needed. Perhaps this is the case for Undertaker's human host.
Some people interpret it as a sort of Who Wants to Live Forever? thing. Since Undertaker, at least in character, appears to be functionally immortal, who knows how long he's existed? Perhaps wrestling is just something he's taken up recently (as much as it can be recently to a force of nature) to amuse himself/pass the time.
A related question comes from the time when Vickie Guerrero booked Undertaker vs. Edge in a TLC match for the World Heavyweight Championship, with the stipulation that if Taker lost he would be banished from WWE forever. Undertaker had the match won on several occasions, but the continuous strategic interference of La Familia enabled Edge to overwhelm Taker and win the day. More important than that, Taker was never seen signing the contract to agree to the stipulation. Yet the Undertaker would never show up anywhere to protest this ban. Infact, the next time he reappeared was at SummerSlam, after Vickie had voluntarily rescinded the ban and booked Edge vs. Taker again, this time in a Hell in a Cell match, due to Edge's infidelity. The question is, why would the demonic grim reaper who is so frightening and fearsome that Satan answers to HIM with 'sir', accept being banned for life under circumstances which were clearly unfair and which he never signed off on to begin with, when he's got the powers to easily render said banishment meaningless?
Perhaps it's because, as an ancient force of nature, the Undertaker wasn't amused by unfair wrestling and just went home until his friends decided to play fair again.
