In Harvest Moon DS and Cute DS, the Friendship Pendants would act on a random selection of individuals to boost heart levels a little. If you equipped the Large Friendship Pendant before you leapt into bed at night, it would raise the affection/friendship levels of a random selection of individuals by 2 points. It did NOT raise every individual's levels but if you repeated this ritual nightly, it probably would select every individual at some point in time, thereby helping your character to achieve or remain at maximum popularity with the entire Valley.
In Island of Happiness, the Friendship Pendants appear to operate in a similar fashion. The Large Friendship Pendant appears to raise the heart levels of a random selection of individuals by a couple of points nightly. Even when I have ignored an individual, his/her heart levels rose gradually provided I equipped the Large Friendship Pendant nightly.
Another variable here is Festival Attendance. I never miss a Festival and that is a way to gain points with individuals. The combination of regular Festival attendance and equipping the Large Friendship Pendant is an effective one.
The only reason I am writing this post, however, is to state that the Friendship Pendants do not appear to affect the friendship levels of any of the Subsidiary Characters at all. It is only through regular conversation and the giving of preferred items as gifts that I have been able to raise the heart levels of any Subsidiary Character.
I would be interested in confirmation of this from other players, but in four years on the Island, equipping the Friendship Pendants RELIGIOUSLY, the Subsidiary Characters with whom I have interacted little remain at low heart levels.
Note that, until a Main Character has reached Yellow Heart level or higher, that character WILL leave the Island if you fail to perform the requisite actions that keep him/her on the Island regularly.