var of "to be", "summer", name
昰 is a rare variant character with no single clear conceptual center. It is recorded as an alternate form of 是 (to be, this) and also as a variant for 夏 (summer), as well as a name kanji. Its meaning depends heavily on context and usage.
昰 is a variant character form. It appears as an alternate way of writing 是 (to be) and 夏 (summer), but the exact historical development and original component relationships are uncertain.
The top 日 (sun) over 正 (correct) can suggest the sun being directly overhead, marking the height of summer, or the idea of something being correct and present (to be).
For ゼ, imagine a Zen monk meditating under the sun (日) in the correct (正) posture, embodying the essence of being: Zen -> ゼ.