Character Analysis
Before we even begin to talk about this "angry boy," you should know that some editions of the play spell his name "Kastrel" so, don't freak out if your copy spells the name differently than we do. By the way, the name means "small, angry, falcon," which is pretty fitting for this obnoxious kid.
When we first meet Kastril, we find out that he's a young, country gentleman who's visiting London so he can "learn to quarrel and live by his wits"(2.6.61). In other words, he wants to learn how to talk trash and fight because he thinks it will make him one of the cool kids. (Apparently, in the early 1600's some young dudes thought it was trendy to run around picking fights. They were often referred to as "roaring boys." (Source: Brian Woolland's notes to Ben Jonson's The Alchemist (Cambridge Literature), 2010)
Kastril is so bent on being a roaring boy that he's willing to pay Subtle to teach him how to win a verbal smackdown.
So, if you read what we have to say about Dame Pliant, you already know that Kastril's her obnoxious bro and he's always bullying his sis. He also wants to make sure that she marries an aristocrat and basically orders her to marry a "Spanish count," even though Dame Pliant's not really into the guy:
KAS. God's lid, you shall love him, or I'll kick you.
DAME P. Why,
I'll do as you will ha' me, brother.
KAS. Do,
Or by this hand I'll maul you. (4.4.43-47)
Oh, that's real nice.
In the end, Kastril gets tricked when Master Lovewit disguises himself as a Spanish count and marries Dame Pliant without Kastril's permission. At first, Kastril's furious but, when Lovewit puts him in his place, Kastril is totally impressed by his new brother-in-law's smack talk skills and decides that he's a-okay with his sister's "match" with Lovewit." (5.5.136)
(No word on what Dame Pliant thinks of the situation, but we bet she's pretty compliant about the whole thing.)