Like other kosoado pronouns, you can separate these words in this way:
- koitsu こいつ
This guy. This one.
- soitsu そいつ
That guy. That one.
- aitsu あいつ
That guy over there. The one over there.
- doitsu どいつ
Which guy? Which one?
These words are Japanese pronouns which can be used to refer to people, guys, girls, whatever, and even individual things.
They behave exactly the same as the pronouns kono, sono, ano, dono together with the words yatsu やつ, that is, koitsu is kono yatsu, soitsu is sono yatsu, aitsu is ano yatsu and doitsu is dono yatsu. In fact, they were originally koyatsu, soyatsu, ayatsu and doyatsu.
Another thing is that these pronouns can be joined with the pluralizing suffix -ra ～ら to form the words koitsura, soitsura, aitsura and doitsura, which would be "these guys," "those guys", "those guys over there", "what guys," etc. It can also pluralize the word when used to refer to things instead of people.
Koitsu vs. Kono and Kore
The difference between koitsu and kono is that koitsu is used for a random individual thing while kono is used for a thing of certain given attributes. The attributes don't matter for koitsu, so it's like saying "this one." We know it's one of a thing but we don't know what it is nor do we care.
The difference between koitsu and kore is that koitsu is used only for individual things while kore is used for the general, abstract idea. You can use kore to say "this won't work" in a general way but koitsu can only mean "this one won't work."
- koitsu wa odoroita! こいつは驚いた！
I was surprised about this one!
- koitsu wa baka desu ne こいつはバカですね
This guy is an idiot, isn't he?
- koitsu wo dou omou? こいつをどう思う
What do you think about this one?
- koitsu ga dou natte mo ii no ka? こいつがどうなってもいいのか？
Don't you care about how this one ends up?
(phrase often used by a bad guy holding hostages at gunpoint)
- soitsu wa ningen da そういつは人間だ
That one is human.
- soitsu wa omoshiroi そいつは面白い
That one is funny.
That one is interesting.
- soitsu wa dame da そいつはダメだ
That one is no good.
- soitsu ni nani wo shita? そいつに何をした？
What did you do to him? (her, that one)
- aitsu ni wa maketakunai あいつには負けたくない
I don't want to lose to him. (that guy)
- aitsu wa ii yatsu da あいつはいいやつだ
He's a good guy. (that guy is a good guy, she's a good guy, etc.)
The word aitsu is often used for guys, "he," "him," etc. and is sometimes used to talk about love interests, "he," in songs and romantic movies, specially in their titles. However, you can use aitsu for women too. Not to forget you can use these pronouns for things too. Look:
- dono juu ga hoshii どの銃が欲しい？
Which gun do you want?
- koitsu こいつ
(said after picking up a gun or pointing to it)
The word doitsu is practically never used in questions. Most of the time it's used in expressions together with the particle mo も to mean "whatever one" or "whatever guy." It's also a rather rare word in general.
- doitsu どいつ？
(basically the only time you'll see this word as a question)
- doitsu mo koitsu mo! どいつもこいつも！
koitsu mo doitsu mo! こいつもどいつも！
This guy and every other guy!
(expression used when mad about everyone doing stupid things)
- doitsu demo kakatte koi! どいつでもかかって来い！
Come fight whichever of you!