Erika in MegaTokyo, while currently a Deadpan Snarker as well as a BadassAction Girl, was a voice actress and Idol Singer in her day, and still can't bear the thought of her fans thinking of her as anything but upbeat and bubbly. Dom calls her out on it by showing that even in the midst of a rampage of rabid fans and excessive taunting by him, she feels a need to keep her smile going.
Grace from El Goonish Shive seems to be veering into Depressed territory, with occasional powerful beings advising some help before she pops on over to Unstable.
Shroomy from Electric Wonderland has a cheery and naive demeanor undermined by depression caused from continuously crushed hopes.
In The Non-Adventures of Wonderella Rita is tied up by a super-intelligent gorilla who boasts that his empathic helmet will give him the insight into humans he needs to take over. After hearing in a story in which her emotions fluctuate while she remains smiling, he's told that human ladies are supposed to keep smiling all the time, no matter how they feel. He responds by throwing away the helmet, and possibly giving up on wanting anything to do with humans.
In Shiniez Ally has aspects of this; she keeps her anxieties to herself to remain strong and at times forces smiles. Part of the story of the comic is Lisa gaining Ally's trust so that she can get to the 'human under the mask.'
Calliope from Homestuck acts very kind and cheerful, but inwardly she's depressed and for pretty good reasons ranging from body image and feelings of personal inadequacy to quite correctly fearing for her life at the hands of her sociopathic brother.
Aradia begins showing shades of Instability once she reaches god tier, and gets more unnerving in Act 6 Intermission 5.
Ollie Pompey of Something*Positive is, according to his uncle, "bottling it all up in a cask of finely-aged denial. His eventual breakdown will be my greatest artistic achievement". To what extent this is true is unclear, as Ollie was definitely performing Obfuscating Stupidity, but he is definitely in severe denial about how abusive his uncle really was, so he still has a dash of this.
Horuss of Homestuck manages to pull off all three types. It's effectively in layers: He admits to be repressing his true feelings by smiling all the time. Underneath that is complete madness, and belying that is an Empty Shell.
The Archangel Michael from Satan And Me puts on a friendly/cheerful facade to get his way.
In Contest Jitters, Jerry clearly has issues with Janet's bodybuilding, but he hides them well enough for Janet to be oblivious to them.
In Evil Plan, it later hints that Computer ( Will) is both this and a Stepford Snarker. He cheerfully makes fun of Dr. Universe/Stan, and is usually pretty cheerful. Then you learn his tragic backstory and how much pain he had to go through. And then you see him crying behind a blue file with a smile emoji on it and your heart shatters.
In The Glass Scientists, Jekyll is the Depressed type. Outwardly, he's a polite, genteel socialite who always has a perfect smile for anyone and everyone likes him, but inside, he's a bundle of insecurities and self-loathing, always terrified that someone will discover the deception.
Mixed Gender Examples
The Order of the Stick give us the Empire of Blood, an entire nation that smiles and steps ultra carefully because committing any minor crime or act of rebellion will result in them being packed off to the Gladiator Games, eaten by a dragon, or worse. Best exemplified in this strip, where a pair of reporters are covering a parade held in the Empire. Within seconds of the female reporter accurately identifying a group of marchers as the Empire's death squad and criticizing the regime, she is shot with a poisoned arrow and replaced by a female member of the death squad. All the while, the male reporter keeps smiling.
This is very prevalent among the depressed characters in Depression Comix, and is represented by a character holding a card with a large smile drawn on it over their mouth.