Mari and Airi are DLC only!!

They've gone and done it. Its an awfully naughty way to sell DLCs but they've gone and done it. The 2 extra hostgirls, Mari and Airi are avaliable for 800MSP each. So that's how you unlock them.

On the bright side, you can now use them on your first playthrough! If they were unlocked through gameplay they couldn't be your first. If you're an offline player, then its pretty awful news.

They will be made available on 27 August with the release of the game.

Source: http://blog.livedoor.jp/maxell011/archives/970422.html

that's quite lame, like the overpriced DLC for im@s.
i'd prefer to have those girls for free even if that would mean to unlock them later in the game.

Ok. My bad.

I was in a hurry and only skimmed the page, making unnecessary assumptions. A more careful reading of the description clearly states

"A character that can be unlocked through clearing a special condition".

The 800MSP DLCs per character, are for those who want Airi and Mari instantly or have trouble clearing the special conditions. They can be unlocked through normal gameplay. The following was also stated

"These two characters have much harder routes."

Implying that the unlock condition might range from clearing the game to archiving difficult conditions during gameplay. If its hard enough, I suspect quite a few will throw down the 1600MSP rather then tear their hair out.

I'll let online wiki guide teach me how to get them XD

Still no news about how to unlock them?

The condition is amount of achievement unlocked, 30 achievement for Iri and 35 achievement for Mari.

So we pretty much have to clear all the default girls to have enough achievements to unlock them.

You don't need to finish all girl to reach that amount of achievement. There are lot of achievement you can unlock during the game such as mini game achievement or job achievement. So, take your time on that.

We need a faq for it...lol! :3

^Indeed!
And seeing as how I'm just now learning to read Japanese... a faq/ walkthrough would be most helpful.