noun
endorser
A person who endorses a check, bill, or other negotiable instrument by signing the back.
小切手の裏書人として署名してください。
Please sign as the endorser of the check.
裏書 is the act of endorsement or the endorsement itself; 裏書人 is the person who endorses.
Compound of 裏書 (endorsement) + 人 (person).