51Kg demon ahhh
when i was in my late teens 18-19 i was 57kg and started working out, i also started eating pretty much anything in between my normal meals(2 bananas, a yoghurt, a chicked filèt etc) Just to get more good food in me during the day( i escalated the amount over time. it worked magic and i ended that year being over 85kg. have to add i was doing my mandatory military service during this year so i got ALOT of excercise, and we went gym after the day was over.
telling u this as a person that struggled to put on weight for years, turns out(for me atleast) it was just eating more of anything really(BIG SURPRISE)
EDIT: i dont know muh about nutrition i just had a rule where i would eat around 200 grams of SOMETHING between my other daily meals pretty much
EDIT: i know this is ALOT of weight in a year but it has to be added that i was getting around 4500 calories a day by the end.