Meeting The Watcher
From your vantage point above you can see the group of convicts below -- you want to drop down and take them out, but now is a good time for us to discuss why you want to do that, because in theory that fight below is one you could avoid... If you wanted to -- which you do not!
As with the previous game in the series, advancement in Arkham City requires you to upgrade your kit, your armor, and get new gadgets, and you accomplish that by earning XP that you can then use to spend upon those upgrades and items. You earn XP from doing lots of things -- completing levels, completing side-quests, discovering clues, pretty much every action that has a formal move to it. You also earn XP for each of the convicts that you best in combat, so fights are not something that we want to avoid -- plus the more you fight now, the sooner you will learn the nuances of battle moves.
So drop down and clean their clocks good!
--== Side-Mission: Watcher in the Wings ==--
After you finish with the thugs, look directly across the street to see a two story building with a grapnel spot on it and you should spot a figure standing on the roof. BEFORE you RB up to the roof to confront them, select the Remote Batarang from your kit using the D-Pad (lower-right corner) and target the figure, launching the Remote Batarang so that its little orange targeting dot is on the upper chest or head of the figure on the roof.
As the Remote Batarang approaches him, the figure will reach out and snatch it out of the air! This unlocks the optional Achievement "Catch" (5 GS) for playing catch with the Remote Bataraing! Note that if you ended up confronting the Mysterious Stranger before playing catch with them, you will have three more opportunities to unlock this as the game story progresses, but getting it out of the way now is an idea!
With that minor Achievement managed, go ahead and RB up to the roof so that you come face-to-face with The Mysterious Watcher -- and watch as he carves a symbol in the roof and disappears -- you then use your Detector to scan the symbol, which engages the Batcomputer -- and shows you the opening screen for this side-mission, Watcher in the Wings. And how cool is that??
At this point we are just supposed to keep an eye out for him and scan any clues he leaves -- we can do that... When you exit the scan you will see that two new Bio's have unlocked -- The Joker, and this Unknown character. Normally we do not cover the unlocking of the Bio's but as it is The Joker this is a special case -- but be aware that as you complete different missions, side-missions, and secrets in the game you will be unlocking all sorts of content like Bio's, character statues and the like. These are part of the collectable system but not part of the Riddler Collection -- but we do include a ticklist of all of the items in these separate systems elsewhere in the guide.
You will also have completed your very first Level-Up in the game with this so go ahead and choose your first upgrade now!
Triggering our first Side-Quest: Watcher in the Wings
A check of the Waynetech screen shows that you have possible upgrades for Catwoman as well as Combat Upgrades for yourself -- here is the thing about this... You will probably want to go with an upgrade for yourself, and if you do I don't blame you, really... But the Catwoman Parts that will be approaching ahead get harder as we go, so I ended up deciding that it would make my life a LOT easier to upgrade her armor. Which is what I did, going with the Combat Armor v1.0 (the second selection -- the first was for ballistic upgrade).
Before you go inside here if you look up you will see the chopper from which the radio broadcast is coming from -- instead of going inside to deal with Harley why not break line-of-sight with the chopper then snag its skid and use it to take a tour of the city! Putting Detective Mode on will show you all sorts of interesting things -- including Riddler Riddles -- which we are not doing on this play-through... You will also get a LOT of information from the radio while you ride along -- pay attention to it.
Right then, now that we have had the grand tour it is time to deal with Harley!