What does クーデレ Kuudere mean?

A kuudere is a character who is outwardly cool, composed and unexpressive, and inwardly attached. The type is real and well populated. The word is on much thinner ice than the type is.

How it is built

クール is the English cool, borrowed. デレ is clipped from the doubled mimetic word for going soppy over someone, the same element in tsundere and yandere.

So the construction is an English adjective plus a Japanese fragment, assembled on the pattern of two existing words. That is a legitimate way to make a word, and it is worth noticing that the pattern is being followed rather than the language generating the term independently.

The honest position on the family

Two of these terms have solid currency in Japanese fandom. Beyond those, the list expands into a long taxonomy covering every temperament, most of it circulated by English-speaking fans, some of it invented for charts explaining the charts.

This one sits in the middle: more widely used than the far reaches of the list, less established than the two anchors. Presenting the whole family as standard Japanese vocabulary overstates the case considerably, and this site said so on the yandere page before writing this one.

Why the type outlives the terminology

Because the character is genuinely useful. A person whose feelings are real and undisplayed creates a specific kind of tension: the audience is asked to read evidence rather than statements, and every small gesture carries weight it would not carry from an expressive character.

That works regardless of what it is called, and it long predates the vocabulary in fiction everywhere.

What English would call it

Nothing compact. Reserved describes manner. Repressed diagnoses. The strong silent type is close, carries a gendered assumption, and belongs to a different set of stories.

So the coinage filled a real gap in fan vocabulary, which is the best case for it. The weaker claim, that it is a Japanese word English has borrowed, is the part worth handling carefully: it is a word built from borrowed parts, largely by the people who needed it.