How does Olivia from Twefth Night fit into this category: " An admirable person is the one who thinks of others, not himself/herself"?
I don't think she fits into this category at all. Let us remember that the first we here of Olivia is that she has taken a vow to remain single out of grief for her brother's death:
The element itself, till seven years' heat,
Shall not behold her face at ample view;
But, like a cloistress, she will veiled walk
And water once a day her chamber round
With eye-offending brine: all this to season
A brother's dead love, which she would keep fresh
And lasting in her sad remembrance.
Even before we have met her, there is an element of excess in this vow as she allows her emotions to get the better of her and to plunge her into a state of misery that is maintained for seven years. However, the next time we see her, she, in spite of the solemnity of this vow, falls in love with Caesario, Duke Orsino's messenger, even though it is Orsino's love that Caesario was sent to plead. Once she has been infected with this love, she proceeds to act incredibly selfishly, abandoning Malvolio in his time of need and only thinking of her own interests in pursuing a relationship with Caesario. There is no sense, therefore, in which Olivia is a character that could be said to think of others.