(Where are the HiiAi Stans)
I know nobody cares but according to this one Japanese site which may or may not be accurate, Hiiro's and Aira's birthflowers are pretty and I think they match....

(Continuing because I *need* to get it out even if it's not a big deal)
Hiiro's flower is Tweedia (Also called Blue Star). In hanakotoba, it means "Happy Love" or "Believing Heart". It's leaves are said to be heart shaped and the color is blue (hence similar to Aira's indigo)

Aira's flower is Crossandra (Firecracker Flower). In hanakotoba, it means "Inner Beauty" and "Friendship". Apparently, the seed pods explode which is why it is considered as an explosive flower. But this flower describes Hiiro so well.(Commonly orange but red variety also exists)

So basically, Aira's and Hiiro's birthflowers (not birthmonth flowers) describe not themselves but the other person perfectly. Idk. I like to think of it as Hiiro having been born with Aira's flower and Aira being born with Hiiro's flower or some cringe stuff like that.

Another thing I found interesting is that Tweedia is a natively South American flower while Crossandra is a natively South Asian flower. They are from opposite sides of yhe world pretty much like Hiiro and Aira who seem to live in completely different worlds at first lol