Helen

Skill comparison
Attack Skill Ultimate skill Passive skill
 * Helen gives a short damage bonus to the whole team, including herself, with a visual effect to let you know when.
 * This is a good time to use Ultimate Skills. She pairs well with Khan, in particular.
 * The damage bonus increases by 20 percentage points per weapon tier. A 4-Star weapon will give an 80% bonus, a 5-Star weapon will give 100%.  It's a strong ability at high levels!
 * "Skill Damage" items will not improve Helen's power.
 * A single attack travels down the screen in front of Helen, about 60% of the width of the path.
 * It does low-average damage, but you probably want to build it up with items because Helen has other abilities that activate at the same time. When you are controlling Helen, you can use it during her damage boost, so it's better in play than it is when you just read about it.
 * Everyone heals 10% of their health when any of your three characters completes an Ultimate. (They won't heal at the start of the animation, so in Kingdom Defense they might die before the healing happens.)
 * This might reduce your healing if the party has a lot of Heal When Dodging items, but in general it helps. Characters dodge less in Duels, so it is especially good in Duels.

Special Item skill
 * High-level Helen hats will add time to the stage timer when Helen uses her Ultimate. It does not activate for other Squad members.
 * The first time Helen uses her Ultimate, the timer increases by the amount in the item description, but the bonus gets cut in half every time you use it. For example, Helen might add 13 seconds, then 6 seconds, then 3 seconds, then 1 second, then nothing.
 * This works in Challenges and Friend Quests, but it does not  help with Clan Raids or Kingdom Defense. You also shouldn't need it in Duels.
 * This is an incredible power. Nobody else can do it, and high-level missions are more limited by time than by health.  At high levels there will be missions you can only complete if you have this power.

Game Mode Comparison
Difficult Adventures or Challenges
 * Pros:
 * Attacks penetrate to hit multiple enemies.
 * Damage bonus helps a lot against bosses.
 * Her hat's power is necessary to complete the most difficult missions, and it removes some of the pressure in medium-difficulty missions.
 * Cons:
 * Her damage bonus is less useful than a real attack when you're just clearing away swarms.
 * Healing doesn't help you get a 3-star mission completion.
 * Helen hats don't help increase her damage, and Explosion Resistance doesn't help you get a 3-star mission completion.
 * If you are unlucky, you will need to spend a lot of resources combining items to get a good Helen hat.
 * Overall value: Very High, sometimes Required!

Easy Adventures or Challenges (for farming, grinding, or daily quests)
 * Pros:
 * Attacks penetrate to hit multiple enemies.
 * Damage bonus helps against bosses.
 * If you bought Auto-Ultimate at Level 3 VIP status, Helen might let you farm extra-difficult missions, but most players probably don't have this. (It costs $50 USD.)
 * Cons:
 * Damage bonus does less for your clearing speed than a real attack would.
 * Ultimate Skill bonuses are not useful. (If you bought Auto-Ultimate, at Level 3 VIP status, this might change.)
 * Even if you have Auto-Ultimate, she won't clear missions quickly, which again limits her value.
 * Overall value: Low (unless you bought Auto-Ultimate)

Duels
 * Pros:
 * The healing and the damage bonus both do a lot for you in battles.
 * Her Ultimate, while not strong, does at least hit most of the screen.
 * Cons:
 * Her hat doesn't help against players, so you might want to change her hat when you aren't running missions with her. This reduces her Set Bonus, sadly.
 * You might also have her equipped with items that cause bullets to vanish, which doesn't help against players. You may want to do a lot of equipment-changing to use her in duels, and having two sets of equipment for her does consume some resources.
 * Overall value: High