The Katana is considered a mildly beginner friendly bike. It *is* a 600cc in-line four. It is pealy and powerful. But more than that, it is also heavy. It is close to 500 pounds with a tank of gas.
While I have never ridden a Kat, I seriously doubt it is terribly nimble compared to a much lighter sport bike.
Certainly people have learned to ride on Katanas. But that doesn’t mean it is a good idea. Just beacause a sport bike (such as the Kat or the ZX-R) are not the current crop of road going track specials does not mean they are friendly bikes to learn on. They *will* bite you when you make a mistake. For your first bike, avoid inline fours. The only exception to this is the GSXF650, which frankly is still a very heavy bike (it replaces the Katana).
The twins are much nicer engines to learn on. If you simply must have the power, then go with one of the 650 twins (SV, Ninja). I seriously question anyone who tells me that the 250 isn’t fast enough. Fast enough for what exactly? Even with a 300 pound rider the 250 will out accelerate 90% of cars on the road and reach “do not pass go, do not collect $200” speeds.
The ninja 500 is probably the single most under-rated sportbike in modern history. It can accelerate to 60mph in under 4 seconds. It can corner better than most riders can push it. Yes, the guys who spend all night looking at gixxer centre-folds will look down their nose at you. But that doesn’t mean they’ll be able to leave you in their dust. And more importantly, it doesn’t mean they’ll have more fun than you.
One last point, before you decide on a bike, talk to your insurance broker / agent. Depending on who you are insured with and where you live, there could be a huge difference in cost between insuring a supersport (like the gixxer), a sport-touring (like the katana), and a standard (like the Ninja 500). Especially as a new rider who need to take those things into consideration. Many places around here simply won’t insure a new rider on a supersport.