Semi-Diagonal Alphabet
Python 3, 59 bytes



lambda l:[' '*i+'%c '%(i+65)*-~i for i in range(ord(l)-64)]


Try it online!

Python 3, 61 bytes



lambda l:[' '*i+-~i*(chr(i+65)+' ')for i in range(ord(l)-64)]


Try it online! (link to pretty-print version)