Hello, I was researching Filn-tan and I came to realie it's basicaly impossible to find any info online aside from a few articles and some art that has breached containment, your blog was the only english resource I could find that even mentioned her by her name. Do you have some more art of her by any chance? Also, I know she's designed by Akio Watanabe, but do you know if there's any relation to the circle 夢先案内回覧板 / artist ひろひろき? There's this dj I've found around the internet (the art in your banner seems to come from there https://otakurepublic.com/product/product_page_4460773.html?ref=tag_page&type=tagpage_product) and I guess it could just be an unrelated doujinshi, but the artstyle and obscurity of the character makes me wonder if there was some official affiliation. Huge NSFW warning for the circle and artist mentioned just in case I guess lol. Anyways, I can't find any sort of actual relation between the artists mentioned online, and hit a dead end since there's barely any info even in japanese, and the name of the characters makes it difficult to google too, so any info is appreciated.
There doesn't seem to be any relation other than one moe lover (Hiro Hiroki/ひろひろき) encountering another moe lover's moe creation (Akio Watanabe) and drawing her because she was cute. They were both popular moe otaku artists in the 2000s and were frequently contacted for work to draw cute girl characters, and have crossed paths before (such as illustrating for MOSAIC.WAV) but I don't see any direct relation with the two regarding Filn.
Her birthday was November 1st. She had an older sister, Faln (ファル), but she seems to have been less popular. Not surprising, since Filn was the poster girl.
To find more information about Filn (the character), you should look for information regarding Filn (the website). All the information and archived pictures I found were from looking through the archived site and official blog links. There's a short wiki page on the Japanese Wikipedia as well.
Filn created a demo video and a 3D promotional anime for the site, with Filn being voiced by Momoi Haruko and Faln by Rikimaru Noriko. There are two 3D promo videos, both the same visually, but Promo Video A is a regular anime with dialogue, and Promo Video B is the anime overlayed with a conversation between the two. There was also a web radio show, but I'm not sure how many episodes there were. The president of Filn uploaded what seems to be the first one though.
As for art, there are a few posted on Pixiv (NSFW warning for one image if you're logged in and have R18 enabled), and the art I use for my blog was grabbed from Danbooru. There was also a Christmas illustration exhibition showcasing art done by contributing SNS users, but only the thumbnails are archived for the one I linked above. There seems to have been more exhibitions, as two users (1, 2) uploaded their artwork onto Pixiv and neither match any of the thumbnails in the above link, but I don't feel like looking for the links to those exhibitions, sorry. Here are some images I found from looking through some Internet Archive links of the official blog and site. There's two of Faln as well. I hope this has been helpful! Good luck with your research!