I'm a bit muddled with which contrapositive and converse we're talking about here (and also I'm guessing you meant
f2(x)=x2?).
Basically we have 4 statements:
1. f is increasing => f² is increasing
2. f² is not increasing => f is not increasing
3. f² is increasing => f is increasing
4. f isn't increasing => f² isn't increasing
(1) and (2) are always either both true or both false, since (2) is the contrapositive of (1) and vice versa. Also, (3) and (4) are always either both true or false, for the same reason. (3) is the converse of (1). We've shown that (1) is false earlier in the thread, and your example here has shown that (3) is false (assuming we're only looking at the region
x>0, since x² is decreasing on
x<0!) and so (2) and (4) are both necessarily false.
The contrapositive for the converse (i.e. (4)) is "if f isn't increasing then f² isn't increasing". Well, as you've just shown,
f(x)=−x isn't increasing on the interval
x≥0, but
f2(x)=x2 is increasing on this interval, so this is false.