pawsplay
Hero
5 Con could very well represent that the character is played by an actor who wanted more salary. Or lacking a will to live. Etc. It doesn't have to mean "sickly," and in fact, it's not really low enough to qualify as sickly. If the player has a 5 Cha and then expects to be treated like a charismatic character, they will be disappointed. If the player is then disappointed, they can only blame themselves. I think much more harm can be done trying to tell players how to roleplay their characters than players roleplaying in a fashion that doesn't quite mesh with their ability scores.
Ability scores are not measurable scales of any real thing. They are an abstraction in game. They measure nothing more than how often the character succeeds at stuff, how many skills or languages they know, how much they can lift, et cetera. If a character has Dex 18, it doesn't matter if they're a Deadly Viper Assassin or if they are Jar Jar Binks, what matters is their AC.
What precise qualities cause a character to succeed or fail at certain tasks is a story rationale. And ultimately, the player has greater authorial control over their character. Perhaps the Charisma 5 barbarian is a swell guy and a natural leader of men, but due to the events in the story, makes enemies left and right and contends against wily villains who easily see through his motives. Maybe he's cursed to die friendless. Maybe the Int 13 fighter/wizard has a brilliant mind but simply lacks the drive and imagination to excel at wizardry.
When a rogue gains evasion at 2nd level, they don't suddenly develop a new super power. They just suddenly become more likely to survive fireballs and dragon breath. Presumably it's an evolution of their previous abilities. An Int score is no less abstract than evasion.
In the real world, Intelligence is devilish to define, Charisma is intangible and socially bound, and Wisdom involves as much luck and pluck as it does willpower or alertness.
Ability scores are not measurable scales of any real thing. They are an abstraction in game. They measure nothing more than how often the character succeeds at stuff, how many skills or languages they know, how much they can lift, et cetera. If a character has Dex 18, it doesn't matter if they're a Deadly Viper Assassin or if they are Jar Jar Binks, what matters is their AC.
What precise qualities cause a character to succeed or fail at certain tasks is a story rationale. And ultimately, the player has greater authorial control over their character. Perhaps the Charisma 5 barbarian is a swell guy and a natural leader of men, but due to the events in the story, makes enemies left and right and contends against wily villains who easily see through his motives. Maybe he's cursed to die friendless. Maybe the Int 13 fighter/wizard has a brilliant mind but simply lacks the drive and imagination to excel at wizardry.
When a rogue gains evasion at 2nd level, they don't suddenly develop a new super power. They just suddenly become more likely to survive fireballs and dragon breath. Presumably it's an evolution of their previous abilities. An Int score is no less abstract than evasion.
In the real world, Intelligence is devilish to define, Charisma is intangible and socially bound, and Wisdom involves as much luck and pluck as it does willpower or alertness.