For me, Ren has a very distinct character voice, so usually I put in a few words from the Rendog language post and it does like 70% of the work if it's nothing too serious. Hit the dialogue with the Rendogification beam! I know there's a post of people Rendogifying DSMP quotes and it's hilarious. Add a "my dude" before/ after his dialogue and it'll usually work. And maybe pepper in some "get in line"s. Or "outrageous!", "geez", "freakin'".
For HC Ren / Ren in general, I put him as like... a friendly, enthusiastic, charismatic guy who's very supportive of his friends and he's always eager to help. He is kind and sees the best in everyone. He's also very dramatic and unashamed of his dramatic tendencies (singing out loud, posing, twirling). And of course he flirts. He flirts with everything. He's probably flirted with someone's houseplant. He wants everyone to get in line for him. He makes a lot of inappropriate jokes (check my #the rd difference tag) which is greeted with complete silence.
But underneath that, I think he's a guy who tends to look down on himself / self deprecating and he is soooooo self sacrificial. He's gonna be good at something then go "I've never been good at anything 🥺". And he is genuinely so eager to sacrifice himself at all times; he'll probably throw himself in front of his friends to save them. And I think Ren is terrified of being lonely which is why he's clingy. If you leave him to his devices for too long he'll literally burn / blow up his base (Last Life, HC 7). Ren is fantastic with words, he's optimistic and naturally good at bringing people together, but he's also just a doggy who wants company.
Ren is quite talkative and he can ramble about random topics forever. But he also occasionally asks questions to his friends so they can contribute to the conversation. And I think he's fantastic as a storyteller because not only is he a master at "yes, and", he also likes including his friends and subtly inviting them to roleplay with him.
Ren is highly sentimental and remembers a lot of memories. He's not afraid to go sappy and sincere and vulnerable, but then again there's his fear of being a burden. Ren has a tendency to trail after competent, snarky, and mildly intimidating people (insert Ren's greens: Doc, Iskall, Martyn, False, etc) and enjoys being lovingly bullied and lovingly used as a punching bag. He lovesssss being a pathetic lil guy. He loves acting sad and soggy. And he kinda loves being beheaded and beheading his friends.
Ultimately, I think Ren greatly values company and loyalty, and always desires a tightknit group of people to belong to, maybe to even lead (Knights of the Square Table, Dogwarts, etc). And he wants to be useful and not be a burden, leading to his self sacrificial tendencies. He is so eager to serve and offer his services to the rest of the server— in HC, he likes selling / providing food in the early days (HC 4 he made a farm, HC 9 Gigapies) and then proceeds to build infrastructure for everyone to enjoy (HC 6 Hermit Railway Network), or come up with detailed "game" systems (HC 5 Hermitron (?) and HC 9 Hermit Quests). In that way, despite his theatrics, Ren is quite practical and down to earth. Give the man something to manage and he'll be happy.
I don't read a lot of HC fics nowadays but I don't think I've seen like, really severe mischaracterisation of Ren. Probably because Ren RPs a lot so he can RP as a lot of characters which decreases OOC-ness, maybe? Ren's a bit pathetic but he's not a coward, he will literally die for his friends. And while Ren might be a bit of a "derp", he has decent strategies (sometimes) which is mostly him accumulating resources + building a secure fortress (evident in Life series).
For Life series Ren, it's similar but more... guarded and cautious. Still desires a tight knit group of allies (usually his top priority). Still loves gathering his resources and building his defenses. Is kind and forgiving to the point he's taken advantage of (this behaviour decreases in latter series). And he loves his allies with all of his life and would die for them.
Oops, this got long lol. Hope this was helpful! :D