Formal:
* Detective (if they are professionally involved in solving mysteries)
* Investigator (for someone who conducts inquiries, regardless of profession)
* Enigmatist (someone who is fascinated by puzzles and riddles)
* Analytical (someone who enjoys using logic and reasoning)
Informal:
* Mystery buff (someone who enjoys mysteries as a hobby)
* Puzzle solver (someone who enjoys figuring things out)
* Sleuth (someone who enjoys investigating and uncovering secrets)
* Curious mind (someone who is naturally inquisitive and eager to learn)
* Problem solver (someone who enjoys finding solutions to difficult situations)
Humorous:
* Sherlock wannabe (someone who is aspiring to be like the famous detective)
* Nosey parker (someone who is overly curious about others' affairs)
* Amateur sleuth (someone who solves mysteries as a hobby)
The best term to use will depend on the specific context and the nuance you want to convey.