Hello. I'm yuanqiao, Japanese Sensui & Itsuki fan for 15 years.
I would like to write about Sensui & Itsuki.
Thank you.

Itsuki means “tree” in Japanese.
I’ve often heard that the color of his hair (teal) originated from the leaves of trees.
There are other three reasons, I think.
First, the kanji of 樹 is closer to tree as “life” than as “material”.
Let’s look at some other examples.

tree sap/樹液
bark/樹皮
wooden bed/木製のベッド
wooden dish/木製の皿

Second, Itsuki means demon (yokai). In many countries, people has told legends of demons dwelling in the depths of forests. A forest create an aura of mystery.
Basho Matsuo (1644-1694) made famous haiku (one of Japanese-style poem) about the mystery of trees.

Suma Temple -
hearing a silent flute
under dark trees
(Sumadera ya fukanu fue kiku koshita yami)(1688, Summer)

In fact, Itsuki and Sensui met for the first time in the forest.

Third, 樹 is usually pronounced as “ki”. So, “Itsuki” is a special pronunciation.
斎 is the same pronunciation as Itsuki. 斎 means xerophagy. So, “Itsuki” has a mysterious image.

I take some examples again.

斎場/funeral hall
斎宮/vestal virgin, imperial princess serving at Ise Shrine
斎戒/purify oneself

Now we can imagine many possibilities.
Mr. Togashi probably named Itsuki wholeheartedly.
In addition, the first thing I think of about Itsuki’s image is a bower for rest.
Sensui could have found rest for his soul through Itsuki, I think.
In a subspace (aka ; the world of shadows,pseudo-space, Limbo) , they look like Michelangelo's statue of Pieta in Saint Peter's Basilica.
[link]ieta

I'd like to go to Italy to see that. Of course, I'd like to see other Sensui & Itsuki fans who I have never met too!!