Make a short sound that is clearly about them, such as their name, a nickname or an inside joke, and send it as an audio file they can set. A ringtone is a gift they hear every day, so the words matter more than the length.
In Ringoz, type their name and a line only you would write, pick a style that fits them, and save the audio file to send. Ringtones made for someone else are common: in our ringtone trends report, about four in ten were made for a contact rather than for the maker's own phone.
Related questions
What are good personalized ringtone ideas?
Their name plus a line in their voice: a nickname, an inside joke, a catchphrase.
How do I make a ringtone with someone's name?
Make a short clip with their name in it, then assign it to their contact.
How do I set a different ringtone for each contact on iPhone?
Open the contact, tap Edit, then Ringtone, and choose the tone.