I mean, besides just "don't". That would make for a short list. Let's assume that a mascot character is like getting 85 or more on Metacritic: The designers don't get to eat if there isn't some adorable chirping nuisance making the game ever so slightly worse.
That said, there are plenty of mascot/pet characters in RPGs (mostly the Japanese kind) that can turn out genuinely pretty neat. The trick is to follow the rules below:
"DO have your cute mascot character secretly be a diabolical space tyrant."<br>
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I'm just saying it helps if the game pulls a misdirection ploy and make the mascot character Space Hitler. Mana Khemia doesn't skimp on the ludicrousness, even for a JRPG.
"DO have your cute mascot character be occasionally sagacious (and a brutal debuffing wild card)."<br>
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Riki is definitely underrated, given how much is going on underneath that tribble-like surface. Even if he does have a cutesy speech impediment. Xenoblade's toughest fights are no problem with his status effects.
"DO have your cute mascot character secretly be a sympathetic high-ranking operative for the bad guys."<br>
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See space tyrants, above. If you're going to hide a mole under the player's nose, better use the mascot character they're assuming is some shoehorned-in bullshit for kids.
"DO NOT have your cute mascot character complain about being hungry every five minutes for "levity"."<br>
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Regardless of your appreciation of this thing in combat, as being FFIX's Blue Mage gives him something of an edge, having him interrupt the main quest/story constantly with his culinary nonsense doesn't really make for an engaging video game experience. See also: Tales of Innocence's Coda.
"DO have your cute mascot be an invaluable weapon in combat."<br>
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If the mysterious waif insists on taking her adorable pet everywhere she goes, she better be prepared to hit monsters with it. See also: Grandia's Fluffy and Tales of Eternia's Quickie.
"DO have your cute mascot character be a total badass dog."<br>
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I feel this is self-explanatory. Obviously dogs can be cute (sup, Rorie) but they can be fierce as heck too. See also: Persona 3's Koromaru, Shadow Hearts Covenant's Blanca, FFVIII's Angelo, The Mabari War Hound from Dragon Age or the player-named dog of Secret of Evermore.
"DO NOT have your cute mascot character be a total wuss dog constantly shown up by a goddamn future bee."<br>
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Way to represent Canis lupus familiaris, King.
"DO have your cute mascot character be a total badass... pig?"<br>
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Well, Hans doesn't really do much besides fly around and make fighting in Valkyria Chronicles' version of WW2 seem extra magical and fun. My first choice for this category is, instead, the precociously awesome Hamlet of Illusion of Gaia.
"DO NOT have your cute mascot character live in a character's afro (and then become a sexy cosplayer in the sequel)."<br>
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Because seriously, what the hell Final Fantasy XIII?
"DO have your cute mascot character be a miniature giant space version of a regular animal."<br>
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Or to put things even more succinctly, just have Boo in your game. Mass Effect knows what's up.
"DO have your cute mascot character be a sentient wisecracking droid."<br>
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To hell with lists, I'm gonna do what I always dreamed of: I'm gonna write that sitcom about the sassy robot. (See also: Chrono Trigger's Robo, Anachronox's PAL-18)
