why do a majority of stories choose Issei's harem over their own harem?

there are no shortages for girls but Rias, Akeno and the other members of Issei's harem are chosen systematically?
what creativity and inventiveness!

why ntr Issei by pairing Irina with your character?

make sure you dont go crazy with harem members some people just keep adding more and more and those character never get introduced because the serious got dropped so keep it under control because the more you add the more the plot thins and stretches and gets out of your control

you dont have to put (honestly) after every sentence it’s annoying

You should add church trio into harem

Man im not really a fan of try hard goody two shoes gentleman wannabe. Issei for all his faults atleast breaks that cliché and acts honestly.

Damn right you are, a DxD story needs to be a harem it may not be an official rule (which doesn't exist in the first place) like the commenter below me said but it needs to be a harem nonetheless.

Making a DxD story that isn't a harem is like going all the way to a restaurant only to order a glass of water. Its not satisfying and its not right.

Yes add them

yes add the church trio and good story to read.

to my knowledge there is NO rule which specifies that Dxd must be harem. sometimes a simple pairing is good too.

to complete what the first person below me says, Irina is a childhood friend Issei and the first girl to have kissed him.
she would be better off with Issei. same thing for Asia.

finally only if you plan to use Issei in this fanfic.

not a fan of seeing Akeno, Koneko in oc's harem.

Rias, Akeno, Koneko, Xenovia are the most overused quartet in every Dxd story and often paired with someone other than Issei.

Give him the church trio. Also interested to see what his abilities will be exactly.

Would rather have Asia and Irina be with Issei IF you plan on using him for this fic

To your question. Yes add them.

For Asia and Irina: sure. There’s probably some way to explain them liking Levi. For Asia, it’s a simple as her meeting Levi instead of Issei. For Irina, maybe a bit trickier. Either they also have a shared past like she does with Issei or something happens that causes her to like him in the Kokabiel Arc.