guileless, sincere, honest
愫 centers on a pure, unadorned inner state: being sincere, honest, and without guile. It describes a heart that is straightforward and true.
愫 combines 忄 (heart) with 素 (plain, unadorned), suggesting a heart that is plain and without artifice. The exact historical development is uncertain, but the component relationship is clear.
The heart radical 忄 beside 素 (plain, unadorned) paints a picture of a heart that is simple and unembellished—someone guileless and sincere.
For ソ, imagine a person so sincere that they sew a plain white heart onto their sleeve: sew -> ソ, and the plain heart shows their honesty.