Okay, so I actually get why this is tricky—writing for Piccolo specifically trips people up because his whole vibe is 'minimalist alien warrior' and folks default to either silent stoic or weirdly chatty. The mistake I see most often is having him over-explain feelings or drop long speeches. Dude's a Namekian who spent decades alone in the wilderness; he's not gonna monologue about his emotional state. I think the key is in what he doesn't say. Let his dialogue be functional—short, direct, often a command or a factual observation—and let the 'reader' character's internal monologue or the scene's action fill in the emotional subtext. Like, instead of him saying 'I am concerned for your safety,' he'd just move to physically position himself between the reader and a threat, maybe grunt 'Stay back.' The authenticity comes from the contrast between his terse words and the protective (or reluctantly mentoring) actions. Also, remember his humor is deadpan and often unintentional. He wouldn't tell a joke, but his blunt assessment of a chaotic situation could be hilarious. Pull from his canon interactions with Gohan and Pan, not just the big fights but the small moments. That quiet, grumpy mentorship is the core of his later character, way more than the early 'I am the demon king' stuff.
Another thing that helps me is reading the dialogue out loud. If it sounds like a natural response from someone who values efficiency over social niceties, you're probably on track. Don't be afraid of silence in the scene. Let pauses do a lot of the work. And for the love of god, avoid pet names or terms of endearment unless it's a very, very specific established dynamic—even then, he'd more likely use the reader's actual name, if at all. The tension in a Piccolo x reader scene often lives in the gap between his actions (which can be incredibly caring) and his words (which are sparse and practical). That gap is where the reader's imagination and emotional response kicks in, and that's where the real connection in the fic happens.