The following characters become unlocked when you complete the indicated task.

Unlock Alisa:
By the Fall of Year 2 or later raise Nathan's friendship to 2 White Flowers and then walk to the Mountain Summit between 10:00 p.m. And 3:00 a.m. On a sunny day.

Unlock Dirk:
Walk from the low Konohana mountain area (bare-hand fishing area) to the entrance Konohana area (where Sheng and Kana live) while the weather is sunny all day. Note: you cannot ride a horse between the areas.

Unlock Mikhail:
After 5th of Fall on a sunny day, your town's mayor will arrive at your house at the beginning of the day with Mikhail to introduce him.

Unlock Nathan:
On 3rd of Winter he will be introduced to you by the mayor. He can then be found at the church in Bluebell.

Unlock Oracle:
In Year 2 walk from the Bluebell low-mountain area to Bluebell mid-mountain area after 8:00 p.m.