detain, fasten, halt, stop
留 centers on keeping something in place: detaining a person, fastening an object, or bringing movement to a halt. The core idea is preventing departure or change.
留 is thought to combine 田 (field) with a phonetic element that also suggests stopping or staying. The exact historical development is uncertain, but the character has long been associated with detaining or halting.
Picture a field (田) with a fence-like top that stops animals from leaving. The whole shape looks like something fastened down to halt movement.
For リュウ, imagine a detained person named Ryu who is stopped at the field: Ryu -> リュウ, and he cannot leave.
studying abroad
to stop; to stay (e.g. the night); to cease; to put an end to
overseas student; exchange student
reservation; putting on hold; deferment; withholding
staying behind; remaining; residual
absence; being away from home
detention (usu. during investigation); imprisonment; poundage; custody
care-taking; house-sitting; house-watching; staying at home
residing (esp. abroad); staying; living
stationing (e.g. of troops); garrison
repeating a year (at school); staying in the same class for another year
reserving; withholding
custody; detention; confinement
karuta; traditional Japanese playing cards, esp. hyakunin isshu karuta or iroha karuta
stop (bus, tram, etc.); station; stopping place
internment; detainment; detention
registered mail; registered post; registration (of mail)
remaining in office; staying in office; remaining at one's post
general delivery; counter collection; poste restante
clasp; latch; catch; fastener
police cell; place of detention; jail