One reason why Rosalind and Celia's friendship can be seen as the perfect friendship is due to their loyalty towards each other. We especially see Celia's loyalty for Rosalind. Celia especially disapproves of her father's actions and wants to distance herself from him, actions such as usurping Rosalind's father, Duke Senior, and snubbing Orlando after he wins against Charles in the wrestling match. More importantly, when Celia's father banishes Rosalind from the household, Celia decides to go with her. We especially see Celia's dedication to remain with Rosalind when she boldly solicits her father to banish her as well as Rosalind, as we see in her lines:
Pronounce that sentence then on me, my liege:
I cannot live out of her company. (I.iii.85-86)
Beyond their loyalty and the fact that they claim to be inseparable, another sign of their close friendship is their honesty. For example, when Rosalind as Ganymede tells Orlando to meet her while she pretends to be Rosalind and he is late, Rosalind bemoans the idea that he may not truly be in love. While others might have tried to comfort Rosalind by assuring her she knows he is in love with her, Celia teases her by saying, "Yes, [he is true in love] when he is in; but I think he is not in" (III.iv.27). Celia's approach is actually a much more honest approach. It's doubtful that Celia really believes Orlando is not in love with Rosalind; instead, Celia's approach is leaving room to point out the irony of the situation. Rosalind, at the moment that she is crying and thinking Orlando does not love her, is actually not being Rosalind; she is being Ganymede instead. So of course it is unlikely that Orlando would be in love enough with Ganymede play acting as Rosalind to show up on time. Hence, Celia's honesty perfectly fits the situation. But more than that, Celia's response also leaves the possibility open that he may not be in love with Rosalind, making her response a much more honest and supporting response, despite the teasing manner her response is delivered. Celia's honesty on even such a delicate issue further shows just how loyal she is to Rosalind.