I'm borrowing the idea of the 'Info Vault' from Random Usagi.
If you guys want to contribute or add any link, feel free to leave a comment below ^^
Official website and SNS:
The lead actors:
Haru is currently a member of the J-pop group Bullet Train.
Where to watch:
The series has still not been released on an official platform for international fans. In Japan it is temporarily available on TVer, TBS Free and Netflix. If you download the NekoCap extension (and a VPN if you don't live in Japan), you can watch it on TVer and TBS Free with English captions. See Borax's thread for more info.
Official OST:
The theme song is "U&I" by Hi-Fi Un!corn. The single is available on many streaming platforms, such as Spotify or YouTube Music. Check out the complete list here.
The official MV with scenes from the series:
Shuto, who plays Yokouchi in the series, is one of the vocalists of Hi-Fi Un!corn. Here is a live performance of "U&I" with Shuto as the main focus:
The original manga:
"Kimi ni wa Todokanai" (also known as "I Cannot Reach You") is based on a Japanese manga of the same name written and illustrated by Mika. In Japan it's released by MFC Gene Pixiv Series and published by Kadokawa. The English edition is published by Yen Press.
Drama merchandise:
You can find the official merch on TBS's online shop: https://shopping.tbs.co.jp/tbs/shop/goods_dvd/drama_kiminai.
Shooting locations:
You can check out the shooting locations used in each episode here (the blog is in Japanese): https://asikotz.com/entertainment/drama/kiminai/#outline__3_1.