It was shown when Bookman told Lavi to use the wood seal to clear the clouds (Three Akuma were hiding behind the clouds). Lavi stated that he forgot he could use it to affect nature directly, and as he remembered it the question mark turned into a kanji that he used right away. If seen more carefully the kanji for the wood seal can be seen throughout the manga whenever Lavi summons his seals long before the use of the actual seal itself (e.g. during the use of the heaven seal). At the moment just before its first use the seal can be seen just behind Lavi's head and had never transformed from a question mark in the first place.There are many different translations of his name, such as Ravi, Rabi, Labi. But it is found in the D.gray-man Omake from episode 61 that the correct spelling is Lavi.