系 means lineage, system and is a JLPT N1 kanji. It has 7 strokes and uses the thread radical (糸).
Stroke order · 7 strokes
On'yomi (音読み)
ケイ
Kun'yomi (訓読み)
—
Radical
糸 · thread
Strokes
7
How to remember 系
A lineage strung together on a single thread (糸) — a family line or system. 理系/文系 — science track or humanities track, a major fork in Japanese student life. 日系 means of Japanese descent.