The Story Behind the Forging that the Girl Doesn't Know (少女の知らない鍛冶場の裏話) is a special short story given to those who purchased volume 15 of DanMachi in Gamers.
Summary[]
Finn was shocked to learn that Lili had ranked up. The Pallum had stopped by the Goibniu Familia's workshop to pick up his Fortia Spear which he had given them for maintenance. Before handing it over, Goibniu informed him of Lili's achievement and that just the day before, she had gone to ask for a order made weapon.
The news of a fellow Pallum's growth made Finn extremely happy, even more so than his own growth. In particular, he was surprised to learn that it was Lili in particular who had ranked up.
Finn then proposed that he use the remaining materials from his weapon to make Lili's weapon. Goibniu had no problem with it but asked if he was sure. Those were rare and fragile materials that didn't even exist in Orario, and Finn himself wouldn't get anything in return. Finn explained that he owed the Hestia Familia a lot, and while he didn't think that he would be fully paying them back with this, he wanted to do it to show his good faith. More than anything, he wanted to offer the materials as a gift to his brave comrade.
He finished asking him to keep it a secret with one eye closed and a mischievous smile, causing the unfriendly God to let out a rarely seen chuckle. The material used to create Lili's weapon was Oathtree Walnut, so it ended up being named Sciurus Walnut, which meant "Squirrel's Walnut Shooter", in reference to that connection.