Since there are no Power Berries in Harvest Moon DS you will start with the maximum 100 points of Strength and 100 points of Fatigue. The tools that you use will deplete your Strength until it reaches 0, then the Fatigue begins to count up to 100. Once you have 0 STR and 100 FAT your character will pass out for the rest of the day. If you pass out you will loose half your money! You can see your current STR and FAT values by equipping the Truth Bangle.
To counter the stamina depletion you can equip items from the Kappa and Goddess line of accessories. They will help you manage your stamina so you don't pass out. The Kappa accessories will help with your Fatigue Points and the Goddess accessories will work with your Strength Points.
The beginning items you can purchase have little affect on your STR and FAT, but you can purchase better ones once you have the 1st version of the accessory. The best Kappa and Goddess items are cursed items, hidden in the 3rd Mine.
Goddess Hats
Van's Shop, 200,000 G to 500,000 G
The Goddess Hats will prevent you from loosing Strength when you use a tool.
Kappa Hats
Van's Shop, 200,000 G to 500,000 G
These work the same as the Goddess variety, but they prevent your Fatigue
points from increasing when your STR is at 0 and you use a tool.
Goddess Earrings
Sprite Shop, 400,000 G to 700,000 G
These are incredibly helpful! If you equip the earrings and stand outside,
you will slowly regain your Strength points. Don't equip these earrings in
your Decoration slot if you have 0 STR and are playing the boy version
of Harvest Moon DS, else the opposite will occur and you will drain your
remaining FAT at an alarming rate. The earring work correctly in Harvest
Moon DS Cute.
Kappa Earrings
Sprite Shop, 500,000 G to 900,000 G
It's easier to just keep an eye on your Strength and use the Goddess Earrings
when necessary, but if you ever find yourself with 0 STR and a high amount of
Fatigue Points, equip these earrings and stand outside. Your FAT will decrease
back down to 0 the longer you stay outdoors.
Witch Earrings
3rd Mine
You can only find the Witch Earrings hidden in the ground inside the 3rd
mine. The earrings have the combined powers of the Kappa and Goddess earrings,
but you must remove the curse from the accessory before using it.
Goddess Broaches
Van's Shop, 50,000 G to 10,000 G
Since there are no Power Berries you can't permanently increase your Strength
Points, but you can temporarily do so if you equip a Goddess Broach. Once you
remove the accessory from your Ornament slot your STR will drop down to a
sub-100 level.
Kappa Broaches
Van's Shop, 50,000 G to 10,000 G
The same as the Goddess variety, but these increase the amount of Fatigue
Points you have to work with before you black out from exhaustion.