The newest tidbits of information regarding Dragon Ball's next movie, Dragon Ball Super: Super Hero, has surprisingly resulted in a lot of new hype surrounding Piccolo. Thanks to new concept art of Piccolo's home -- apparently Team Four Star's frequent jokes regarding Piccolo's homelessness in DBZ made it to Akira Toriyama -- fans are delighted to see the iconic Namekian get his dues, simple as they may be.
Piccolo as a character has, regrettably, fallen to the wayside during Goku's never-ending quest to get stronger. Although to be fair, Piccolo has had more important things to do, namely raising Goku's children and now grandchildren. His surrogate fatherhood of Gohan is an amusing running joke, but it's also a fantastic dynamic between two of the franchise's most famous characters.
Piccolo's pseudo-adoption of Gohan starts the way any healthy surrogate father and son relationship does -- he murders the boy's father and then abducts Gohan himself so they can train to fight aliens. At first, Piccolo's decision is strictly pragmatic, having just witnessed the child Saiyan's hidden power and understanding they'll need all the help they can get to defeat the Nappa and Vegeta. And if Gohan can help Piccolo take over the world after that, well, all the better.
The young half-Saiyan quickly wins over the didn't-know-he-was-a-Namekian-yet, as Piccolo grows protective of Gohan even while training him. After a long, bloody battle, Piccolo ultimately sacrifices his life to protect Gohan. Using his dying breaths, he thanks Gohan for being the first friend he's ever had. Ever since this moment, Gohan and Piccolo have had the most profound bond of any two characters in the Dragon Ball franchise.
Dragon Ball Z spends an absurd amount of effort getting Goku offscreen as frequently as possible, using death, injury, disease, tardiness, and even plain indifference to keep the mighty Saiyan uninvolved unless absolutely necessary. This leads to much of Gohan's formative years being spent with his bonus dad, affectionately nicknamed "Mr. Piccolo." Even after Gohan and Goku spend a year together in the Hyperbolic Time Chamber, when they get out, Gohan goes straight to Piccolo to ask for clothes like his, right in front of Goku.
Piccolo's peak dad energy occurs again during the Cell Saga when the furious Namekian berates Goku for throwing Gohan into the ring with Cell. Even though Goku turned out to be right about Gohan's hidden power, Piccolo was also correct that Gohan is not a warrior at heart, a factor Goku had never considered. In the Buu Saga when Goku and Vegeta are both busy being dead again, Piccolo takes over mentorship and Fusion training of Goten and Trunks, treating them similarly to how he treated Gohan at their age. Roughly, exasperatedly, but from the heart.
While it's been disappointing to see Piccolo drift away from the life of a warrior, his strange talent for wrangling half-Saiyan toddlers is a credit to his character growth. Considering his own father only had him to get revenge on Goku, it's delightfully ironic that Piccolo now babysits Goku's granddaughter in Dragon Ball Super. Piccolo even proudly warns Goku that Pan may cry if someone other than Piccolo holds her, indicating dedication, not obligation, to caring for Gohan's daughter.
The gross, slimy Namekian egg fell pretty far from the tree in this instance, as Piccolo's caring nature has made him the best father in Dragon Ball by a wide margin. Now that Gohan is a grown man with a family of his own, his relationship with Piccolo more closely resembles a friendship, with Piccolo now in grandpa territory thanks to his relationship with Pan. Dragon Ball Super: Super Hero has aged Pan up a few years, so it's entirely possible that the mailbox spotted outside Piccolo's home is just there so Pan can send him letters. Either that or Goku finally got a job and needed somewhere to send the checks for child support.