Question: why is izuku midoriya called deku?
Answer: Izuku's nickname, "Deku", was given to him by Katsuki when they were little. "Deku" is another reading of his name 出久, and Bakugou uses it to mock Izuku, as "Deku" can mean "one who can't achieve/do anything". Later, Ochako finds the nickname very inspiring because "Deku" gives off a feeling of "I can do it".