ought, by all means, necessarily
須 centers on necessity and obligation: something that must be done, by all means, or ought to happen. The idea of 'necessarily' connects the given meanings of obligation and inevitability.
須 originally depicted a person's face with hair or beard strokes (彡) beside the head (頁), suggesting a bearded face. Over time it was borrowed to express necessity or obligation, likely through phonetic loan.
The left side 彡 looks like three strokes of hair, and the right side 頁 is a head. Imagine a person whose beard is so long it must be trimmed—it is necessary, by all means, to cut it.
For ス, imagine a sign that says 'You must sue the barber!' because the beard trim was a disaster. Sue -> ス, and it is something you ought to do.
indispensable; essential; requisite; required; compulsory; necessary
Susanoo (deity, younger brother to Amaterasu); Susanoo-no-Mikoto
small teapot
absolutely (ought to); by all means
fin whale; finback (whale); razorback whale (Balaenoptera physalus)
gosu porcelain
essential amino acid
required entry (e.g. in online forms)
mandatory attribute; required attribute
essential condition
indispensable case
required subject; compulsory course
essential fatty acid
spout of a teapot
broadnose sevengill shark (Notorynchus cepedianus, species of circumglobal cow shark with 7 gill slits)
moo shu pork
Yokosuka Line
ridged whale meat used to make whale "bacon"
blue whale (Balaenoptera musculus)
Beyrich's slit shell (species of sea snail, Pleurotomaria beyrichii)