An exact solution is not traceable... The closest katana is a Nodachi (大太刀) without a Tsuba 鍔 (Handguard) and with a blue/light yellow decorated Tsuka 柄 (Hilt).
Image from Sakura no kuni.
As you mentioned in your question it could also be a shirasaya 白鞘.
A Shirasaya actually contains two plain wooden parts. The Saya さや (Scabbard) and the Tsuka. These parts are used to store a katana blade and prevent it from corroding.
The Shirasaya Tsuka is actually not made to be used in combat because of the missing Tsuba 鍔 (Hand guard). I also do not think that a storage tsuba is fixed strong enough to the blade to use it for fighting.
Image (Sakura Shirasaya) by Bahr3DCG from DeviantART.
Furthermore there is also a Shikomizue 仕込み杖 that general also do not have a tsuba.
A Shikomizue is actually a sword hidden in a stick. Propose of such a stick is to hide the sword from the public because it was not allowed to openly carry a sword.
Image (Shikomizue Katana) by bernardfokke from DeviantART.
In addition they are mostly decorated because it should look nice and unobtrusively like the tsuka of Monohoshizao.
Anyway it is rather be a Nodachi because of length and it is also slightly curved. Just without a hand guard.