Bring a pot of lightly salted water to a boil. Cook ramen in boiling water, stirring occasionally, until noodles have cooked through but are still firm to the bite, 2 to 3 minutes. Drain.
Heat olive oil in a large skillet over high heat. Place tofu on one side of the skillet and pepper and onion on the other side of the skillet. Cook and stir tofu until browned on all sides, 1 to 3 minutes. At the same time, cook and stir onion and pepper until caramelized, 1 to 3 minutes.
Stir ramen noodles into skillet; stir to combine noodles, tofu, onion, and pepper. Pour plum sauce and sweet and sour sauce over ramen mixture; cook and stir until well-combined and heated through, 2 to 5 minutes.
