The text is an 11x11 square, which made me think to check the diagonals. From top-left to bottom-right reads
TWOINIIMAMN, which means nothing, but from top-right to bottom-left reads
USEPIINLINE - "use pi in line".
I took the 3rd letter of the 1st line, the 1st letter of the 2nd line, and so on, using the digits of pi. This gave me
NOWINCOLUMN - "now in column". Repeating the process with the columns - the 3rd letter of the 1st column, the 1st letter of the 2nd column, and so on - yielded
Nicolas Budig and Schmalls, with a little help from the OP, found the next step:
Using the Fibonacci sequence, splitting the double-digit numbers into single-digit numbers, take the nth number from each row. The exact sequence is 1,2,3,5,8,1,3,2,1,3,4, and this yields
TWOUPSFROME - "two ups from E".
This means we need to
Find each occurrence of the letter "E" and take the two letters immediately above it. This spells out
SQUAREBRFIVDIAGONS - "square bottom-right five diagonals".
From there, spaniol6 and Joe found the final step:
Create a 5x5 square using the letters in the bottom-right corner, then highlight the letters on the diagonals, like so:
Who are you, mystery hero?
"I AM BATMAN"
Thanks to everyone who helped crack this, and thanks Keelhaul for making such a well-constructed puzzle.