Hikaru ga Chikyuu ni Ita Koro...... (2011-2014).
High school student Koremitsu Akagi finds himself attending the funeral of Hikaru Mikado, who had died in an accident during Golden Week. He's not sure why he's there, as he only exchanged a few words with Hikaru at school.
However, Hikaru doesn't seem to be completely gone from the world, as he comes back as a ghost in order to fulfill a promise to Aoi Saotome, his fiancée. Since Koremitsu is the only one who can see him, he asks him to carry out his promise in his place.
Complying with his request, Koremitsu tries to fulfill Hikaru's promises to the rest of his "girlfriends," helping them overcome past misunderstandings and repair broken relationships. But the situation surrounding Hikaru's death is shrouded in mystery, as it's uncertain whether or not he truly died in an accident...
I found Jenayah's query on a similar question at Manga: male student dies, comes back as a ghost only seen by his female friend, they hunt for other ghosts and uncover a hoax and borrowed their search string.
ghost boyfriend "only * can see" girlfriend site:myanimelist.net/manga worked for me.