The Footnote in my edition of The Prince edited by Quentin Skinner identifies this ruler as Ferdinand the Catholic (r. 1479-1516), King of Aragon. In Chapter 21 Machiavelli explains his admiration for Ferdinand and his style of rule, especially as concerns his church policy.

We have in our time Ferdinand of Aragon, the present king of Spain. He can almost be called a ‘new prince’, because his fame and glory have raised him from being an insignificant king to being the foremost king in the Christian world. At the start of his reign he attacked Granada—·the Moorish kingdom in southern Spain·—and this campaign laid the foundations of his power. He proceeded quietly at first, with no worries about being interfered with: he kept the barons of Castile busy thinking about the war and not planning any changes ·inside Spain·, and they didn’t notice that by these means he was increasing his prestige and his power over them. He financed his army with money from the Church and from taxes, and through that long war he built a military establishment that has since brought him honor. Further, under cover of religion he embarked on greater schemes, with pious cruelty hunting out the Jews in his kingdom and expelling them; a pitiful state of affairs brought about by an extraordinary act. Under this same ·religious· cloak he attacked Africa, invaded Italy, and now has attacked France. Thus, he has always planned and acted on a grandiose scale, keeping his subjects’ minds in a state of amazement and anxiety about what was going to happen next. And his actions have followed one another so quickly that there has never been a quiet time in which men could work steadily against him.

The Prince, ed. Quentin Skinner and Russel Price (Cambridge: Cambridge Press, 1988), 76-77.

The substance of Machiavelli's praise for Ferdinand follows from the King of Aragon's duplicity and use of religious rhetoric to purse political objectives. In The Discourses on Livy, one of Machiavelli's lesser known and less cynical works, he discusses the practicality of religion for the Romans, and how they always used religious rhetoric to conceal a pragmatic policy choice.

When reasons showed the Romans that something had to be done, they undertook it, notwithstanding the fact that the auspices were unfavorable, but they gave them a different meaning with such suitable words and methods that they did not appear to be doing so with any disrespect to religion.

The Discourses on Livy, tr. and ed. Julia Conaway Bondanella (Oxford: Oxford Press, 2008), 59.

Additionally, because the chapter that Ferdinand is subtly mentioned in is entitled "How Rulers Should Keep Their Promises," Machiavelli's evaluation of the King of Aragon must be considered through that lens as well. The substance of the quote you mention is based on the premise that much of Ferdinand's success came from his tendency to misrepresent his desires and goals. Earlier in this chapter, Machiavelli discusses the veritable archetype of the dishonest Renaissance politician, and how it served him politically.

I must mention one recent case: [Pope] Alexander VI was concerned only with deceiving men, and he always found them gullible. No man ever affirmed anything more forcefully or with stronger oaths but kept his word less. Nevertheless, his deceptions were always effective, because he well understood the naivety of men.

A ruler, then, need not actually possess all the above mentioned qualities, but he must certainly seem to."

The Prince, 62.

Much more can be written regarding Machiavelli and his political project, but I think the context of his immediate admiration for Ferdinand's methods stem from the monarch's adherence to practical results versus religious or moral concerns.

The question of how seriously Machiavelli took these prescriptions is another matter of historical debate, with some historical intellectuals, including Jean Jacques Rousseau considering the work a satire. One explanation is offered by French historian Felix Gilbert, who characterized Machiavelli's philosophical project as the early search for a science of politics. He has written:

To the religious of his [Machiavelli's] age and of the following centuries his teaching--especially his proposition that man must choose between the rules of political activism and the precepts of Christian morality--were thought to be machinations of the devil...But the individual thesis which he propounded would hardly have provoked such a furor had they not formed parts of a vision of politics, relevant and valid. Machiavelli expressed what men were slowly coming to realize: it is impossible to establish one permanent social order which mirrors the will of God or in which justice is distributed in such a way that it fulfills all human needs. Machiavelli clung to the idea that politics had its own laws and therefore it was, or ought to be, a science; its purpose was to keep society alive in the ever-moving steam of history.

Felix Gilbert, Machiavelli and Guicciardini: Politics and History in Sixteenth Century Florence (New York: WW Norton, 1984), 199.