堅
ケン, かた.い, がた.い, きん
strict, hard, solid, tough, tight, reliable
JLPT1 G7 S12 F1049
As per
臤
# (adhere) +
土
earth → tightly adhering earth → *hard*; *solid*; *firm* → *serious*; *formal*.
[Full Kanji Etymology List]