Whether it's a player character or an NPC, people seek out ways to make their characters different. And from different classes, equipment, skills, personalities, and many more, we have many tools to do so in tabletop role-playing games. However, I want to add one more to the mix called “adding a signature skill”, and I hope someone out there finds it useful.
The origin of this technique was from a campaign I was a part of where all of the players were some flavour of spellcasters. Each character got a small twist to one spell 3 times in a period. I'm fairly sure it recharged after a long rest. One example was that they could use the shield spell on someone else during their reaction. Spells are often the same no matter who casts them. The hope here was to change things a little a bit and by doing so make each character more unique in combat as well. Another character had their ice spells enhanced with non-damage related improvements such as taking away movement or being able to seal doors.
These small kinds of changes can go a long way to help distinguish characters. We have other tools, as I mentioned earlier, but it's nice having another one. Sometimes you want to play the same class, but also want them to be their own character. It's that personal touch that helps. It's not in a rulebook, and the odds running into someone with the exact same character decrease drastically. Adding some new options can also be a lot of fun.
Adding a Signature Skill
The idea here is simple. Each character gets one signature skill, which is similar to the spell I mentioned earlier. Where it differs is that it doesn't need to be a spell. It can be some other unique skill not related at all with magic. In fact, it probably won't be if a character has no talent with magic. It's worth pointing out to magic based characters that it doesn't need to be based on their spells though. They can often get so caught with spells that they miss this option. It's an important distinction. The point is to help differentiate the character and make them even more unique. It could be a utility skill, it could be non-magic related, it could be related to their knowledge of magic, or whatever else they want and you are willing to allow.
Still, I find placing a restriction on how often it can be used in the session helps prevent balance issues due to this addition. It's an extra bit of spice, so don't go eating the whole bag was the ideas. It also helps prevent this from turning into getting a feat for free. There is nothing wrong with giving players a free feat or having them come up with their own. I've seen it work wonders. However, what makes this different is the limitation placed on it. This tends to cut down on potential problems, and let's you be fairly significant. I also find that generally avoiding combat enhancements works well, though some non-damage related ones have worked fine in the past. Some feats help with combat so you can come up with some custom feats instead. If you want to add combat enhancements, I'd advise giving two unique skills just so that non-combat is also being addressed and so that everyone gets a boost. Balance in combat isn't the most important thing in the world but I'd advise a light touch since we are additional potential balance issues on top of what the rules already contain. Alternatively I've also seen this used to help a class that everyone at the table felt was wanting.
One thing to think about is if you want to make the skill completely unique. Should there be one NPC with a similar skill? Should an entire school of mages have it? Or should no other character have the skill? The answer to this question varies a lot so it should be up to the Dungeon Master. However, I do find that there is an upside to being a bit strict and not allowing repeats in a group, not even in new campaigns. That way it forces them to come up with a new concept for a new character instead of falling into an old character. This, of course, requires players who want to play new characters and will be grateful for catching them before they accidentally remade the same character without thinking. There are also story arguments for giving everyone a unique skill that marks them as somehow special and others. They work well for their own reasons, but these reasons aren't typically related to helping make the characters more unique.