Is it possible to have a character using the sling and knives as their weapon of choice and trains for their one hundred year life because it’s more inconspicuous, even though they could use a spear and/or a sword decently? Idk, the character I’m writing is strange, but I don’t want to make it too overboard.
That sounds like someone who’s more of an ambush fighter, or assassin, rather than someone who trained for direct combat. Within that context, the sling seems a little strange, but it does make sense. The daggers and sling are both easy to conceal, so a character could carry both of them on their person, and have access to a ranged and melee weapon. Again, that’s more of an assassin or infiltrator’s approach to combat, rather than a straight up fighter.
Your weapon of choice is more about how you approach combat, rather than a preferred style, or aesthetic choice. Multiple weapons would further refine, or plug holes in that selection.
If your character is supposed to be a sneaky, backstabbing type, then a sling isn’t the weirdest ranged option. If they’re supposed to be a fighter that goes toe to toe with their foes, instead of someone shanking people in a back alley, then you’re probably going to want to trade the dagger for a sword. It’s still possible they’d carry both, and use the daggers clandestinely, while using the sword for open combat. That’s more in the range of plugging weaknesses.
If you have a character who operates clandestinely, (like a spy or assassin), it’s entirely plausible they’ve developed proficiency with a lot of weapons they don’t normally carry. This is also true for any character who has extensive combat training, but aren’t using it currently. Ex: Former soldiers or mercenaries.
Weapons are tools. You bring the tools you need for the job at hand. You may know how to use a much wider array of tools. Weapons of choice tell you how a character prefers to approach a problem. It doesn’t tell you everything about what that character can do. Your character could, very well, be proficient with swords and spears in addition to knives, but they’re only carrying a knife because they’re planning to do something specific, and openly carrying weapons would give them away. Especially if your character is more experienced, that’s not “over the top.”
The trade-off with a more experienced character is, they’re more likely to encounter people who know them, or know of them. If your character spent years training with a sword and developed a reputation as an excellent duelist, then that reputation would precede them. This isn’t over the top, it’s a natural side effect of having a character who’s been active for years, or even decades.
If your character is an assassin, having a reputation, and being recognized is a serious problem. Even if that reputation is for something other than being an assassin. It means, people will remember who your character is after the bodies are found, and remember that you were there. This can easily spiral into a complex problem for your character.
Spies have it worse, because being recognized is an immediate threat, and jeopardizes whatever they’re working towards.
Now, if you have a character who’s working as a thief or spy, and they need to infiltrate a situation where they can’t openly carry a weapon, they might choose not to carry any weapon. This is a complicated decision. The short reason is: If they’re caught carrying a weapon when they shouldn’t, it’s an immediate tip-off that they’re not who they say they are. Combine this with the part where a dagger isn’t going to do much good going up against a guard with a spear, however, a smart operator may be able to talk their way out of situation like that, if they’re found. (Obviously, this is contextual. A character who’s someplace they could have reasonably gotten lost can do this. Someone found on in a secret base under a volcano’s going to have a hard time saying they got lost looking for the bathroom.)
The basic concept is only strange if you say they intrinsically know how to use a sword and spear. If they train on a wide variety of weapons, they’d still be proficient with them, but they might only carry weapons that work for their current goals. That’s normal.