Why is Hikari's Capsule slotted in first despite the fact Cosmos debuted before him? He made the Ultra Capsules! Of course he gets to go first!
It might seem odd that Geed can use Belial's capsule without any problems at the start, considering that Orb struggled to use Belial's power at first when transforming into Thunder Breaster. However, the reason for this is because Orb's cards basically "borrow" the powers of the other Ultramen, so Belial had to give him permission (something he only did out of amusement). The Ultra Capsules, on the other hand, are more akin to batteries, and don't require permission to be used - while he might not be able to use them in rapid succession, Geed can use them whenever he wants, as long as they're charged.
Another factor is the obvious: Geed is Belial's son and already has Belial's genes in him already, hence why he resembles Belial in all of his forms rather than just the one using Belial's power.
Moko's relationship with comedian Takashi Arai is more of a Fridge Heartwarming. Think about it: Takashi is a comedian who has multiple times being abandoned by former partners while Moko is a space pet that was abandoned like Samekujiras, thus making each others as Not So Different.
Zero's involvement in the series may be seen by several few as a stand-in for Seven's 50th anniversary or simply because Belial is in that series. In truth, there is also another key point: Zero is the first Ultra to use the gimmick of combining two different Ultra powers into a single form, that is Strong-Corona (Dyna Strong + Cosmos Corona) and Luna-Miracle (Dyna Miracle + Cosmos Luna) and it also makes his appearance in Geed justified, even if he no longer can use them due to the damage of his Ultimate Bracelet.
As seen in Ultraman Orb, Galactron's Galactron Spark is supposedly a dangerous beam output that can decimate an entire countryside but two models that appeared in Geed made the beam to be less lethal. Of course they were less lethal, their controller (Kei Fukuide) is nearby so they can't afford to endanger him.
That or the fact that Kei just wanted to kill Zero and not sacrificing his own fans or Geed himself. The latter also serve him better in collecting Little Stars/Ultra Capsules.
As of episode 11, Kei proclaims that everything that turns Riku stronger is largely his own deed. When you watch episode 6, you realise that Kei transforms into and attacks the city as Thunder Killer not to chase a Little Star, but instead facing Geed. Riku as well gain the new weapon Geed Claw as a result of his growing strength. Kei's true agenda is indeed making Riku obtaining Geed Claw just to strengthen him!
Some of the Little Star hosts are related to their Ultras:
Alien Pitt Tri Tip/Ultraman Hikari: Both are scientists and if you remember properly, they were at first recognized as threats until you learn their true agenda.
Moko/Ultraman Cosmos: Its species' name is Lunah. What was the name of Cosmos' default form?
Mayu Igaguri/Ultraman Zero: She's the daughter of Leito Igaguri, Zero's human host.
Sui Asakura/Father of Ultra: He's the one who named Riku, making him as a father to the boy, aside from Belial.
Kei becomes a rampaging Pedanium Zetton after absorbing all six Ultra Capsules to his body, which turned him mad as well as a result of excessive power. In other words, he has become overdosed.
How did Zero lose to Belial in the first place as Ultimate Zero, when that form was key to Belial's second defeat? Easy: it was the key to Arch Belial's defeat, a much larger and easier target. Another is the absence of the Ultimate Force Zero heroes, who helped kept Arch Belial busy for Zero to take the shot. Zero lost because the Ultimate Aegis was ill-equipped to face a normal Belial.
Back in the first episode, Skull Gomora ravaged across the city and destroyed Riku's home/workplace. The monster has two reasons for doing so; 1) The Little Star of the week Eri had just been there and 2), It was all a plan engineered for Riku to get into the Nebula House and transforms into Ultraman Geed. For those who watch #11, this serves as a Rewatch Bonus and thus, the destruction of Riku's old home is not a coincidence.
Kei seems to hate Zero to a certain degree, which causes the viewers to see him as a disguised Belial at first until they were revealed to be different person altogether. Since Zero's character in Kei's novel is rather popular (despite being casted as the villain), this makes it clear that Kei Fukuide is form of Tsuburaya's Take That! to fans who grew bored of Zero's constantly reappearing in other Ultra Series medias ever since his debut in 2009.
When Kei watches Zero Beyond's transformation and first fight, he left while saying "interesting". Since his mission is to steal Ultra Capsules to revive Belial, Kei may turn to Zero's Capsule sets as an alternative if Riku/Geed fails to do so.
In episode 21, Kei attempts to attack the Little Star host Gubila and almost killed the monster for real if it weren't for Geed's intervention. If one were to look at previous episode, Kei wanted the Little Star/Ultra Capsules so he can revive Belial's lost body, which he regained sometime before episode 16. As Belial died, the Ultra Capsules have no use and if he continues to perform another Batman Gambit like that, he just only empowering the Ultra instead of collecting it on his own.
The Ultra Capsules' original purpose, other than being an immense tool for good, isn't made clear, but Ultraman Zero Beyond gives some idea as to what the intention was for them: allowing the Ultras to share powers with each other and give one another super forms without fusing together.
Given Kei is driven by a pathological need to serve some kind of purpose to the point the shock of Belial's apparent death robbing him of one caused him to suffer temporary amnesia, it paints his murder of Arie after she outlived her usefulness to him in a rather different light. From Kei's point of view, being a thing with no purpose is a Fate Worse than Death, so he might have seen killing her now that she had no purpose as a Mercy Kill on his part.
Why does Belial keep summoning a dark thunderstorm when he comes to Earth? Ultraman Zero absorbs sunlight through his shoulder armor and uses that to power himself indefinitely. Belial blocking out the sun only gives himself another advantage over his worst enemy.
If director Koichi Sakamoto's words are to be believed (claiming that Geed's takes place in the Showa verse, continuing after Mebius), how come current society no longer believes in Ultras and past attack teams never appear? This suggests that the Crisis Impact caused the population either to no longer believe in, or even remember, Ultramen.
The fact that Skull Gomora's attack in Mt. Mitsuse just to harvest Laiha's Little Star is claimed as the first monster attacks recorded. Just how much about the Ultramen has humanity forgotten in general? Was the entire continuity erased thanks to Belial's evil plot?
The AIB are the only ones who were well aware of what happened, but think about it. An organization run by multiple alien races, just how many aliens out there that are bent on conquering Earth, and what if some operatives slip into their ranks?
We learn this eventually in episode 14 when Alien Shadow Kuruto slips into AIB and took the device used to summon the bio-weapon that his race had made, Zegan. In episode 16, it gets worst.
Zena mentioned that space life form Lunah are known to be abandoned as well. So who is Moko's previous owner, and why did he/she abandon it? Is it because of Moko's potential as a monster weapon like Samekujiras, and if so, just what are they capable of?
May crosses the dark humor, but one has to remember that the show ends with our protagonist forced to commit patricide and no one cares about it since the victim is willing to commit filicide.