... your mercy is much appreciated, Emperor Nero. As for the 'why didn't you summon me immediately' quandary, well. After our big mistake with Lucius, I think a lot of us wanted to avoid fumbling the bag again, so to speak. Not to mention Father Kotomine and Jaguar Warrior seemed like they were looking forward to getting a fellow Monitor back, but... it seems like you don't share the sentiment?
NERO: "Did I ask for your excuses? Or for you to barrage me with questions? And this nonsense… 'War Monitors', 'Origins', 'Priests'… and you all, begging me for information like this. You and this 'Solar Cell' are both causing my head to ache."
She had asked for an explanation, though pointing that out seemed like a poor decision.
Additionally, it seemed like unless she wanted to elaborate further, you may not get any much regarding her manifestation outside of 'she did it', in a manner as natural as breathing. Though her stance regarding the Solar Cell seemed readily apparent.
NERO: "And what's this about worrying about some other Servants? Am I not threat enough? Am I not important enough? My needs are exigent! Did I not declare you my Ensemble? And yet you dance to the tune of others? Should my punishment be harsher?"
CONSTANTINE: "And yet, their 'punishment' so far is simply you joining us. It seems like your mind was set from the beginning, wasn't it?"
Rather boldly, CONSTANTINE spoke up. NERO's expression wavered slightly, her attention focusing intently on him.
NERO: "Mmnn… that 'know-it-all' tone, why are looking as if you figured something out, Constantinus-- or, 'Constantinos', was it?"
CONSTANTINE: "Nothing, you're just a bit gentler than I expected. Or more discerning than you may act. As you said, crimes against the Emperor are a grave offense, and there's no shortage of Servants willing to undercut or slaughter my Masters."
…So this was her being kind? I guess comparatively, people have tried to kill you, so this was 'nicer'…
And, in her verbal tempest, she essentially just vented about her frustrations regarding your actions, and then essentially declared her intent to journey with you.
It seemed like she was someone who trusted her judgement and her judgement alone, for whatever reason. Traveling with CONSTANTINE and KUKULKAN, a king and a god, you felt the most pressure emanating from this Servant alone. While the two of them wore their 'doubt' openly, it seemed as if this Servant was almost devoid of the concept of 'doubt' altogether.
And for good reason- this was an Emperor of the Julio-Claudian Dynasty, from the era where 'Emperor' and 'God' were nearly synonymous. To many, trying to contradict the decree of an Emperor was like trying to argue with the rotation of the sun.
NERO cleared her throat, turning away from CONSTANTINE in a huff.
NERO: "Mm. Well, I have not forgiven them, so they should not feel relieved yet."
NERO: "Ah, yes! Those are the cheers I desire! My mood is already beginning to brighten! If you simply wish for more of my art, then ask, and I will indulge! My catalogue on the Solar Cell is sparse, but grant me the time, and I shall have a thousand scripts worth of beautiful artistry!"
It seemed like the best way to get on her good side was praise. An artist at heart, it seemed as if she could not deny any form of applause.
NERO: "The peak of artistry! Beloved by all! One who takes the stage, and has their crafts praised by their audience, immortalized forever!"
She declared, definitively.
NERO: "Unfortunately, I cannot share the next act, as I do not remember it. I do not recall what came over me when I first wrote it, just that I was truly pulling the artistry from my heart. My 'beautiful memories'. I wish to find them as well. Without them, it is like sailing through a fogged ocean."
NERO: "I was a Servant of the Lunar Holy Grail War... I know that, and I know I had a Master. We fought a good war together, against many opponents. Though now, their faces are all blurs to me. Even his. But I remember something, before our 'ending'."
So it seemed like she didn't have her entire set of memories back either. Perhaps more than the average Servant did when they were restored from an Origin, considering her outburst, but it was still limited.
Her expression grew dour.
NERO: "...I do not wish to talk about it right now, it makes my heart ache and my head hurt. I have just been summoned, and was in a pleasant mood. Am I not allowed my own peace for a moment?"
Abruptly, she expressed her desire to change the subject, moving on quickly. Perhaps that's something she'd be open to sharing later.
NERO: "Aha! Now that is a good change of pace! A splendid, beautiful vision indeed! Though that Avenger is a musician, you say? Then he simply has no choice but to join us! Welcome, my new composer!"
It seems as if AVENGER has been/is being conscripted to join your team.
Well, no point in arguing against an Emperor.