You are defining recklessness way too broadly.
Recklessness is the expression of flaws. Of course it's going to encompass a broad spectrum of them, that's the problem.
Sonic is reckless in that he doesn't care about battle risk and believes he can conquer it with his raw physical talent whenever it presents itself, whatever form it takes.
Understanding the risks, weighing out your options, and when no other option is known fighting in the name your fellow man qualifies as alturism. Recklessness is doing something while not being aware of the risk. This as I said before encompasses all sorts of flaws so long as they're expressed. But anyways, people do not normally think 24/7 about their flaws, they can't be very functional that way. So we have faith that for some reason or another we'll make it through. That doesn't mean if we're about to make a mistake we won't listen to others. But it is this faith in something that allows us to be reckless.
I'm not saying we can't be too faithful. However, I don't think that being faithful to the point where one won't back down even after learning their actions will likely be more costly then helpful, or if its revealed safer/more effective alternatives are available, applies to Sonic very much if at all. It's not as much that he doesn't care about the consequences related to his actions. It has more to do with the idea that it never occurs to Sonic to think of the risk associated with his actions, or he does not consider the risk enough. That's where Sally comes in by encouraging him to think. Sometimes however, when Sally will try to "reason" with him and her "reason" is along the lines of "Its dangerous" or "we cannot assess the risk", it goes pretty ignored by Sonic as it probably would by most people.
It's not because he's far too overconfident, but because ALL the FFs have to go on missions and take risks with the element of the unknown. If not doing something because the risks are unknown were a valid reason to ignore every mission, they probably would not be alive today. Even at the end of Doomsday, Sally had to concede when trying to provide "reason" in this sort of way.
That is a very specific aspect of his character;
Your assuming that even if it were, that Sally is only restrained to that particular flaw of his personality.
Sally chastens him and insists that he needs more than physicality to survive the war, yet is obviously attracted to his physicality.
I don't find that very heartwarming. Essentially she values Sonic most especially as an appliance.
You're telling me that Sonic can't have any more aspects to his personality than that?
No.
He can't be additionally cruel (toward Antoine), jealous (of Griff), too trusting of superficial personality traits (with Ari)?
I'm not saying that Sonic has to have one flaw. I am not saying recklessness IS refferring to one flaw or any flaw in particular. I'm saying its the expression of every flaw. Instead of foiling a particular flaw, Sally has access to all of them. I never said she was the voice of dissent for one specific flaw, the exact opposite actually. As for your examples of other flaws, there's Sonic being gullible about Ari's intentions which Sally also chastises him for. There's impatience, which Sally is dissentful of, she's defended Antoine against Sonic's rudeness (Sonic Boom), Sonic's pessimism (Sonic and the secret scolls), etc.
You're saying that we couldn't have used other characters in these situations to develop more interesting plots that didn't overuse Sally?
If you did you'd cheapen Sally's role to the story as well as the other character in question. Because Sally's supposed to balance Sonic, if everyone balances Sonic then the uniqueness of her relationship with him is destroyed. The FFs also pretty much relinquish the responsibility for her to do so, which protects Sally from being cheapened by them.
Not everyone is too reckless.
I never said that not everyone is too reckless. I said that everyone is reckless to some degree.
That's the boogie-man word every time this argument pops up, "flaws", as if a character/person/whoever is nothing besides the sum total of their most negative personality traits.
That's not true. Sonic's positive traits will give him value to his comrades, so it is important to emphasize those too. But likewise, His flaws will give them the chance to contribute who they are to his life to be valued as persons.
If you make a character who can't interact with people beyond their "flaws", then you're a lazy, incompetent writer. Sonic can have Sally be his foil, but still be Tails' big brother figure, Rotor's best buddy, Antoine's antagonizer, etc. That's called "writing".
That's called "empty writing." You slapped on roles like "brother", "buddy" and antagonizer doesn't have to be a role given to two people with a bond that's very strong. Problem is, you've forgotten to add the substance that'd make those roles beleivable. Why does Sonic value highly Tails as a person? Saying he's the brother figure ignores the fact you're thinking backwards. The basis for valuing the person comes before the role is established.
And, at the end of the day, it's all reading way, WAY too deep into a cartoon show that only had 26 episodes, and not enough time to flesh out every inter-character relationship before it's plug got pulled. So chill out.
How is creating a strong heart component thinking way too much about a cartoon? It's an element that helps the quality of the story, and an element that the story implies exists.