How do you write aiko in Japanese?

Aiko in Japanese can have many different senses in whichever you need it is not clear. It can be a name, which has several different writings itself, like 阿依香 or 娃湖 both reading /ai ko/ and are female names. 愛顧 /ai ko/ as a noun means 'patronage' in all meaning senses like 'partiality, favoritism' , 'support' and 'a customer's patronizing'. 相子 /ai ko/ is another possibility, which means 'draw, tie, deadlock'.

If I assume you meant 'aikou' (the difference is in 'ou' sound which is long 'o') it could be 愛好 /ai kou/ (adoration, love).