Wii Fit Trainer is a dynamic all-rounder with heavy emphasis on patient gameplay and managing resources to condition and cover options from a distance, to then run in and steal games with short, high damage combos using unorthodox movement and fast frame data. They have a lot of answers to a lot of situations, and don't necessarily dominate in any area, but are very versatile and can hold their own against most of the cast with the breadth of options their kit contains.
Piloting Wii Fit Trainer requires a great deal of precision and character knowledge. The most active move they have is 7 frames (not counting Super Hoop, a recovery move with special fall), and most hitboxes are placed in strange and awkward places at that. Header tends to stick out as an obviously good move - an outlier in the kit - and as such many end up relying on it as a crutch, not exploring what the rest of the character has to offer.
Past that, even with an intimate understanding of Wii Fit and how to play into the entire roster, there are glaring holes and weaknesses that simply cannot be ignored:
For the most part, you're going to want to focus all your play around Deep Breathing and when you get it. Calling it the best install in the game is almost understating it. It glues the kit together. It is what makes this very weird and strange character viable or even worth exploring and playing. Typically, the secret is to sit reserved and at a distance. Charge Sun, throw ball out to keep the opponent occupied, and wait for a grab or a forward tilt. At lower percents these keep-away moves don't really send the opponent anywhere so it can be hard to get the ball rolling, but once they start to hit the 30-40 percent range hitting anything that sends outward then starting Deep Breathing usually works while whoever you just hit tries to recover or jump back and hit you.
Once you have Deep Breathing, the floodgates open. Pray to God your opponent doesn't stall it out, as this is the correct course of action to take. You can pretty much hit whatever and find success. Your moves are safer, your general movement is faster, and so on and so on. You really want to take this opportunity to overwhelm the opponent with your faster and safer movement to get anything going. Even something as simple as Up Throw > Up Air does 30 percent.