Code point
U+25E9
Glyph appearance depends on your fonts.
U+25E9
SoU+25E9 SQUARE WITH UPPER LEFT DIAGONAL HALF BLACK is a Unicode 16.0.0 character in the Other Symbol category. It belongs to the Geometric Shapes block.
Choose the representation you need, then copy its value.
U+25E9
9705
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E2 97 A9
25E9
000025E9
%E2%97%A9
\u25E9
\u25E9
\u25E9
\25E9